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
diff --git a/bp2build/cc_library_headers_conversion_test.go b/bp2build/cc_library_headers_conversion_test.go
index 2859bab..264ba6b 100644
--- a/bp2build/cc_library_headers_conversion_test.go
+++ b/bp2build/cc_library_headers_conversion_test.go
@@ -84,6 +84,7 @@
 }
 
 func runCcLibraryHeadersTestCase(t *testing.T, tc bp2buildTestCase) {
+	t.Helper()
 	runBp2BuildTestCase(t, registerCcLibraryHeadersModuleTypes, tc)
 }