Don't rely on transitive dependency for guava test. (#4906)

Author: rlazo

firebase-common/firebase-common.gradle.kts

@@ -63,6 +63,7 @@ dependencies {
     // needed for Kotlin detection to compile, but not necessarily present at runtime.
     compileOnly(libs.kotlin.stdlib)
 
+    testImplementation("com.google.guava:guava-testlib:12.0-rc2")
     testImplementation(libs.androidx.test.junit)
     testImplementation(libs.androidx.test.junit)
     testImplementation(libs.androidx.test.runner)