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/osutil/atomic_test.go

@@ -61,14 +61,10 @@ func TestCreateAtomicReplaceReadOnly(t *testing.T) {
 func testCreateAtomicReplace(t *testing.T, oldPerms os.FileMode) {
 	t.Helper()
 
-	testdir, err := os.MkdirTemp("", "syncthing")
-	if err != nil {
-		t.Fatal(err)
-	}
+	testdir := t.TempDir()
 	testfile := filepath.Join(testdir, "testfile")
 
 	os.RemoveAll(testdir)
-	defer os.RemoveAll(testdir)
 
 	if err := os.Mkdir(testdir, 0755); err != nil {
 		t.Fatal(err)