Clean up code duplication in tests with gradleRunner

Test: ./gradlew room:int-test:room-inc-anno-proc:test
      ./gradlew nav:nav-safe-args-gr-plugin:test
      ./gradlew bench:bench-gradle-plugin:test

Change-Id: I13743a39854ca71fdb8b818a5229dda0d5f43fc5

diff --git a/settings.gradle b/settings.gradle
index 7a80011..6b569df 100644
--- a/settings.gradle
+++ b/settings.gradle

@@ -322,6 +322,7 @@
 includeProject(":internal-testutils-truth", "testutils/testutils-truth")
 includeProject(":internal-testutils-ktx", "testutils/testutils-ktx")
 includeProject(":internal-testutils-navigation", "testutils/testutils-navigation")
+includeProject(":internal-testutils-gradle-plugin", "testutils/testutils-gradle-plugin")
 
 /////////////////////////////
 //