mime: remove test ordering dependency

Backport CL 421442 from upstream.

Original description:

Arrange for tests that call setMimeInit to fully restore the old values,
by clearing the sync.Once that controls initialization.

Once we've done that, call initMime in initMimeUnixTest because
otherwise the test types loaded there will be cleared by the call to
initMime that previously was not being done.

For golang/go#51648

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/483117

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-63ba7dd52f2cc49dab4b70ac81309296a920d4dc
+0411a2733fd468e69f1998edd91e8fe3ba40ff9e
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/mime/type_test.go

@@ -14,7 +14,10 @@ import (
 func setMimeInit(fn func()) (cleanup func()) {
 	once = sync.Once{}
 	testInitMime = fn
-	return func() { testInitMime = nil }
+	return func() {
+		testInitMime = nil
+		once = sync.Once{}
+	}
 }
 
 func clearMimeTypes() {

libgo/go/mime/type_unix_test.go

@@ -11,6 +11,7 @@ import (
 )
 
 func initMimeUnixTest(t *testing.T) {
+	once.Do(initMime)
 	err := loadMimeGlobsFile("testdata/test.types.globs2")
 	if err != nil {
 		t.Fatal(err)