mime: remove test ordering dependency

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

Change-Id: I8bf92b305fc4499337db06113817c9decdc5aedb
Reviewed-on: https://go-review.googlesource.com/c/go/+/421442
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

src/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() {

src/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)