Make navigation-testing Kotlin first

Move the navigation-testing classes to Kotlin to
improve the API (no more mBackStack). Removes the
separate navigation-testing-ktx module, moving
its code directly into navigation-testing.

Update navigation-common-ktx to not rely on
navigation-testing, ensuring a one way
dependency.

Test: tests still pass
Change-Id: I685f51391e3f1f44fe783ed77129b3ea37fccca7

diff --git a/navigation/runtime/build.gradle b/navigation/runtime/build.gradle
index 0775541..1c3c026 100644
--- a/navigation/runtime/build.gradle
+++ b/navigation/runtime/build.gradle

@@ -38,7 +38,7 @@
     testImplementation(MOCKITO_CORE)
     testImplementation(TEST_RUNNER)
 
-    androidTestImplementation(project(":navigation:navigation-testing-ktx"))
+    androidTestImplementation(project(":navigation:navigation-testing"))
     androidTestImplementation(TEST_RUNNER)
     androidTestImplementation(TEST_RULES)
     androidTestImplementation(ESPRESSO_CORE)