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/api/api_test.go

@@ -1146,11 +1146,7 @@ func TestBrowse(t *testing.T) {
 
 	pathSep := string(os.PathSeparator)
 
-	tmpDir, err := os.MkdirTemp("", "syncthing")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(tmpDir)
+	tmpDir := t.TempDir()
 
 	if err := os.Mkdir(filepath.Join(tmpDir, "dir"), 0755); err != nil {
 		t.Fatal(err)