fix(build): assert that more than zero androidTests executed

Sometimes an emulator "runs" tests, but it actually discovered zero
tests, runs zero tests, and still reports success. Lookin' at you API21.

Fixes #11078

AnkiDroid/build.gradle

@@ -207,6 +207,24 @@ task installGitHook(type: Copy) {
 }
 tasks.getByPath(':AnkiDroid:preBuild').dependsOn installGitHook
 
+// Issue 11078 - some emulators run, but run zero tests, and still report success
+task assertNonzeroAndroidTests() {
+    doLast {
+        File folder = file("./build/outputs/androidTest-results/connected/flavors/play")
+        File[] listOfFiles = folder.listFiles({d, f-> f ==~ /.*.xml/ } as FilenameFilter)
+        for (File file : listOfFiles) {
+            String[] matches = file.readLines().findAll { it.contains('testsuite tests="0"')}
+            if (matches.length != 0) {
+                throw new GradleScriptException("androidTest executed 0 tests for " + file.name + " - Probably a bug with the emulator. Try another image.", null)
+            }
+
+        }
+    }
+}
+afterEvaluate {
+    tasks.getByPath(':AnkiDroid:connectedPlayDebugAndroidTest').finalizedBy(assertNonzeroAndroidTests)
+}
+
 apply from: "./robolectricDownloader.gradle"
 apply from: "./jacoco.gradle"
 apply from: "../lint.gradle"