Mark test helpers as helpers.

This enables identifying the correct location of test failures.

from godocs:
Helper marks the calling function as a test helper function.
When printing file and line information, that function will be skipped.
Helper may be called simultaneously from multiple goroutines.

Test: go test bp2build tests
Change-Id: I52430e541494c78f5222c11510a4d273fe205a06

bp2build/python_binary_conversion_test.go

@@ -8,6 +8,7 @@ import (
 )
 
 func runPythonTestCase(t *testing.T, tc bp2buildTestCase) {
+	t.Helper()
 	runBp2BuildTestCase(t, func(ctx android.RegistrationContext) {}, tc)
 }