main: fix race condition in tests

This caused most tests to run the zeroalloc.go test instead of what they
should have been tested, and in turn explains most of the performance
gains of parallel testing.

This commit fixes it by avoiding race conditions. Luckily, no tests
started failing since then due to this.

main_test.go

@@ -72,6 +72,8 @@ func runPlatTests(target string, matches []string, t *testing.T) {
 	t.Parallel()
 
 	for _, path := range matches {
+		path := path // redefine to avoid race condition
+
 		switch {
 		case target == "wasm":
 			// testdata/gc.go is known not to work on WebAssembly