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 ankidroid#11078

AnkiDroid/build.gradle

@@ -207,6 +207,28 @@ 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 {
+        // androidTest currently creates one .xml file per emulator with aggregate results in this dir
+        File folder = file("./build/outputs/androidTest-results/connected/flavors/play")
+        File[] listOfFiles = folder.listFiles({ d, f -> f ==~ /.*.xml/ } as FilenameFilter)
+        for (File file : listOfFiles) {
+            // The aggregate results file currently contains a line with this pattern holding test count
+            String[] matches = file.readLines().findAll { it.contains('<testsuite') }
+            if (matches.length != 1) {
+                throw new GradleScriptException("Unable to determine count of tests executed for " + file.name + ". Regex pattern out of date?", null)
+            }
+            if (!(matches[0] ==~ /.* tests="\d+" .*/) || matches[0].contains('tests="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"