android-interop-testing: add AndroidX Test core in implementation (grpc#12718)

This alignment resolves a version skew issue that caused
NoClassDefFoundError crashes during instrumentation tests on Firebase
Test Lab.

Fixes
grpc#12703 (comment)

Author: shivaspeaks

android-interop-testing/build.gradle

@@ -72,6 +72,7 @@ dependencies {
             libraries.junit,
             libraries.truth,
             libraries.androidx.test.rules,
+            libraries.androidx.test.core,
             libraries.opencensus.contrib.grpc.metrics
 
     implementation (project(':grpc-services')) {