Prevent mock filesystem files being overridden by accident

Bug: 181070625
Test: m nothing
Change-Id: Ib12b5cbe9af980706620d7d7d85bcfe31f36b07f
diff --git a/cc/testing.go b/cc/testing.go
index 6840ef4..d8adc61 100644
--- a/cc/testing.go
+++ b/cc/testing.go
@@ -649,8 +649,11 @@
 
 // The preparer to include if running a cc related test for linux bionic.
 var PrepareForTestOnLinuxBionic = android.GroupFixturePreparers(
-	// Enable linux bionic.
-	android.FixtureAddTextFile(linuxBionicDefaultsPath, withLinuxBionic()),
+	// Enable linux bionic
+	//
+	// Can be used after PrepareForTestWithCcDefaultModules to override its default behavior of
+	// disabling linux bionic, hence why this uses FixtureOverrideTextFile.
+	android.FixtureOverrideTextFile(linuxBionicDefaultsPath, withLinuxBionic()),
 )
 
 // The preparer to include if running a cc related test for fuchsia.