Add a basic test case to the new instrumentation test

However, due to the hard limit (60s) on the timeout during the
presubmit, the test is registered as a postsubmit test for now.

Also implement some helper methods and classes to make writing tests
easy.

Bug: 203483081
Test: atest MicrodroidTestApp
Change-Id: Ic07d776aff5f56421fc43a103a5b54b4a62b451f
diff --git a/tests/testapk/Android.bp b/tests/testapk/Android.bp
index 493fc93..32c47dd 100644
--- a/tests/testapk/Android.bp
+++ b/tests/testapk/Android.bp
@@ -6,7 +6,10 @@
     name: "MicrodroidTestApp",
     test_suites: ["device-tests"],
     srcs: ["src/java/**/*.java"],
-    static_libs: ["androidx.test.runner"],
+    static_libs: [
+        "androidx.test.runner",
+        "androidx.test.ext.junit",
+    ],
     libs: ["android.system.virtualmachine"],
     jni_libs: ["MicrodroidTestNativeLib"],
     platform_apis: true,