Modernize unit tests

[marcprux]

This PR removed the vestigial XCSkipTest.swift files that are auto-generated as of skiptools/skipstone#210.

It also puts back the manual insertion of the test runner which had been removed in 92cb15d:

// SKIP INSERT: @org.junit.runner.RunWith(androidx.test.ext.junit.runners.AndroidJUnit4::class)

For some reason, without adding them manually, all the Firebase tests run against Robolectric or an emulator fail with java.lang.IllegalStateException: No instrumentation registered! Must run under a registering instrumentation.:

runtestFirestore$SkipFirebaseFirestore_debugUnitTest java.lang.reflect.InvocationTargetException
java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:118)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at skip.foundation.ProcessInfo.getTestContext(ProcessInfo.kt:37)
	at skip.foundation.ProcessInfo.getAndroidContextstorage(ProcessInfo.kt:27)
	at skip.foundation.ProcessInfo.getAndroidContext(ProcessInfo.kt:23)
	at skip.firebase.core.FirebaseApp$Companion.configure(SkipFirebaseCore.kt:69)
	at skip.firebase.firestore.SkipFirebaseFirestoreTests.setUp(SkipFirebaseFirestoreTests.kt:62)
	at skip.unit.XCTestCase.beforeTest(XCTest.kt:19)
	at skip.firebase.firestore.SkipFirebaseFirestoreTests.beforeTest(SkipFirebaseFirestoreTests.kt:25)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.RunBefores.invokeMethod(RunBefores.java:33)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
	at org.gradle.api.internal.tasks.testing.junit.JUnitTestExecutor.runRequest(JUnitTestExecutor.java:175)
	at org.gradle.api.internal.tasks.testing.junit.JUnitTestExecutor.accept(JUnitTestExecutor.java:84)
	at org.gradle.api.internal.tasks.testing.junit.JUnitTestExecutor.accept(JUnitTestExecutor.java:47)
	at org.gradle.api.internal.tasks.testing.junit.AbstractJUnitTestDefinitionProcessor.processTestDefinition(AbstractJUnitTestDefinitionProcessor.java:65)
	at org.gradle.api.internal.tasks.testing.SuiteTestDefinitionProcessor.processTestDefinition(SuiteTestDefinitionProcessor.java:53)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at org.gradle.internal.dispatch.MethodInvocation.invokeOn(MethodInvocation.java:77)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:28)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:19)
	at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:33)
	at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:88)
	at jdk.proxy1/jdk.proxy1.$Proxy4.processTestDefinition(Unknown Source)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker$2.run(TestWorker.java:178)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.executeAndMaintainThreadName(TestWorker.java:126)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:103)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:63)
	at org.gradle.process.internal.worker.child.ActionExecutionWorker.execute(ActionExecutionWorker.java:56)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:122)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:72)
	at worker.org.gradle.process.internal.worker.GradleWorkerMain.run(GradleWorkerMain.java:69)
	at worker.org.gradle.process.internal.worker.GradleWorkerMain.main(GradleWorkerMain.java:74)
Caused by: java.lang.IllegalStateException: No instrumentation registered! Must run under a registering instrumentation.
	at androidx.test.platform.app.InstrumentationRegistry.getInstrumentation(InstrumentationRegistry.java:45)
	at androidx.test.core.app.ApplicationProvider.getApplicationContext(ApplicationProvider.java:41)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	... 52 more
JUNIT SUITES 1 TESTS 1 PASSED 0 (0.0%) FAILED 1 SKIPPED 0 TIME 0.19

I don't know why this affects Firebase and apparently none of the other Skip packages, but this workaround seem to fix it for the time being.