all: use T.TempDir to create temporary test directory (#8280)

This commit replaces `os.MkdirTemp` with `t.TempDir` in tests. The
directory created by `t.TempDir` is automatically removed when the test
and all its subtests complete.

Prior to this commit, temporary directory created using `os.MkdirTemp`
needs to be removed manually by calling `os.RemoveAll`, which is omitted
in some tests. The error handling boilerplate e.g.
	defer func() {
		if err := os.RemoveAll(dir); err != nil {
			t.Fatal(err)
		}
	}
is also tedious, but `t.TempDir` handles this for us nicely.

Reference: https://pkg.go.dev/testing#T.TempDir
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

lib/model/testutils_test.go

@@ -86,20 +86,15 @@ func createTmpWrapper(cfg config.Configuration) (config.Wrapper, context.CancelF
 	return wrapper, cancel
 }
 
-func tmpDefaultWrapper() (config.Wrapper, config.FolderConfiguration, context.CancelFunc) {
+func tmpDefaultWrapper(t testing.TB) (config.Wrapper, config.FolderConfiguration, context.CancelFunc) {
 	w, cancel := createTmpWrapper(defaultCfgWrapper.RawCopy())
-	fcfg := testFolderConfigTmp()
+	fcfg := testFolderConfig(t.TempDir())
 	_, _ = w.Modify(func(cfg *config.Configuration) {
 		cfg.SetFolder(fcfg)
 	})
 	return w, fcfg, cancel
 }
 
-func testFolderConfigTmp() config.FolderConfiguration {
-	tmpDir := createTmpDir()
-	return testFolderConfig(tmpDir)
-}
-
 func testFolderConfig(path string) config.FolderConfiguration {
 	cfg := newFolderConfiguration(defaultCfgWrapper, "default", "default", fs.FilesystemTypeBasic, path)
 	cfg.FSWatcherEnabled = false
@@ -116,7 +111,7 @@ func testFolderConfigFake() config.FolderConfiguration {
 
 func setupModelWithConnection(t testing.TB) (*testModel, *fakeConnection, config.FolderConfiguration, context.CancelFunc) {
 	t.Helper()
-	w, fcfg, cancel := tmpDefaultWrapper()
+	w, fcfg, cancel := tmpDefaultWrapper(t)
 	m, fc := setupModelWithConnectionFromWrapper(t, w)
 	return m, fc, fcfg, cancel
 }
@@ -213,14 +208,6 @@ func cleanupModelAndRemoveDir(m *testModel, dir string) {
 	os.RemoveAll(dir)
 }
 
-func createTmpDir() string {
-	tmpDir, err := os.MkdirTemp("", "syncthing_testFolder-")
-	if err != nil {
-		panic("Failed to create temporary testing dir")
-	}
-	return tmpDir
-}
-
 type alwaysChangedKey struct {
 	fs   fs.Filesystem
 	name string