For #9605: Add LintUnitTestRunner gradle task.

This will prevent us from regressing on adding older versions of the
unit test runner.

app/build.gradle

@@ -31,6 +31,7 @@ apply plugin: 'com.google.android.gms.oss-licenses-plugin'
 import com.android.build.OutputFile
 import org.gradle.internal.logging.text.StyledTextOutput.Style
 import org.gradle.internal.logging.text.StyledTextOutputFactory
+import org.mozilla.fenix.gradle.tasks.LintUnitTestRunner
 
 import static org.gradle.api.tasks.testing.TestResult.ResultType
 
@@ -684,6 +685,8 @@ task buildTranslationArray {
     android.defaultConfig.buildConfigField "String[]", "SUPPORTED_LOCALE_ARRAY", foundLocalesString
 }
 
+task lintUnitTestRunner(type: LintUnitTestRunner)
+
 afterEvaluate {
 
     // Format test output. Ported from AC #2401

buildSrc/src/main/java/org/mozilla/fenix/gradle/tasks/LintUnitTestRunner.kt

@@ -0,0 +1,103 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.gradle.tasks
+
+import org.gradle.api.DefaultTask
+import org.gradle.api.GradleException
+import org.gradle.api.tasks.TaskAction
+import org.gradle.api.tasks.TaskExecutionException
+import java.io.File
+
+private val INVALID_TEST_RUNNERS = setOf(
+    // When updating this list, also update the violation message.
+    "AndroidJUnit4",
+    "RobolectricTestRunner"
+)
+
+/**
+ * A lint check that ensures unit tests do not specify an invalid test runner. See the error message
+ * for details.
+ *
+ * Related notes:
+ * - The AndroidJUnit4 runner seems to just delegate to robolectric. Since robolectric non-trivially
+ * increases test runtime, using AndroidJUnit4 in place of RobolectricTestRunner is very misleading
+ * about how the test will change.
+ */
+open class LintUnitTestRunner : DefaultTask() {
+    init {
+        group = "Verification"
+        description = "Ensures unit tests do not specify an invalid test runner"
+        attachToAssembleUnitTestTasks()
+    }
+
+    private fun attachToAssembleUnitTestTasks() {
+        project.gradle.projectsEvaluated { project.tasks.let { tasks ->
+            val assembleUnitTestTasks = tasks.filter {
+                it.name.startsWith("assemble") && it.name.endsWith("UnitTest")
+            }
+
+            assembleUnitTestTasks.forEach {
+                it.dependsOn(this@LintUnitTestRunner)
+            }
+        } }
+    }
+
+    @TaskAction
+    fun lint() {
+        val unitTestDir = File(project.projectDir, "/src/test")
+        check(unitTestDir.exists()) {
+            "Error in task impl: expected test directory - ${unitTestDir.absolutePath} - to exist"
+        }
+
+        val unitTestDirFileWalk = unitTestDir.walk().onEnter { true /* enter all dirs */ }.asSequence()
+        val kotlinFileWalk = unitTestDirFileWalk.filter { it.name.endsWith(".kt") && it.isFile }
+        check(kotlinFileWalk.count() > 0) { "Error in task impl: expected to walk > 0 test files" }
+
+        val violatingFiles = kotlinFileWalk.filter { file ->
+            file.useLines { lines -> lines.any(::isLineInViolation) }
+        }.sorted().toList()
+
+        if (violatingFiles.isNotEmpty()) {
+            throwViolation(violatingFiles)
+        }
+    }
+
+    private fun isLineInViolation(line: String): Boolean {
+        val trimmed = line.trimStart()
+        return INVALID_TEST_RUNNERS.any { invalid ->
+            trimmed.startsWith("@RunWith($invalid::class)")
+        }
+    }
+
+    private fun throwViolation(files: List<File>) {
+        val failureHeader = """Lint failure: saw unexpected unit test runners. The following code blocks:
+            |
+            |    @RunWith(AndroidJUnit4::class)
+            |    @Config(application = TestApplication::class)
+            |OR
+            |    @RunWith(RobolectricTestRunner::class)
+            |    @Config(application = TestApplication::class)
+            |
+            |should be replaced with:
+            |
+            |    @RunWith(FenixRobolectricTestRunner::class)
+            |
+            |To reduce redundancy of setting @Config. No @Config specification is necessary because
+            |the FenixRobolectricTestRunner sets it automatically.
+            |
+            |Relatedly, adding robolectric to a test increases its runtime non-trivially so please
+            |ensure Robolectric is necessary before adding it.
+            |
+            |The following files were found to be in violation:
+        """.trimMargin()
+
+        val filesInViolation = files.map {
+            "    ${it.relativeTo(project.rootDir)}"
+        }
+
+        val errorMsg = (listOf(failureHeader) + filesInViolation).joinToString("\n")
+        throw TaskExecutionException(this, GradleException(errorMsg))
+    }
+}