test-systemd-tmpfiles.py: create global temporary directory

For some unknown reasons, the temporary directory created by the test
below is not removed:
```
 # Test the case that a valid symlink is in the path.
 label = 'valid_symlink-deep'
 test_content('f= {} - - - - ' + label, label, user=user, subpath='/deep/1/2', path_cb=valid_symlink)
```
To keep /tmp clean, let's create the global temprary directory.

test/test-systemd-tmpfiles.py

@@ -28,6 +28,7 @@ except AttributeError:
     sys.exit(EXIT_TEST_SKIP)
 
 exe_with_args = sys.argv[1:]
+temp_dir = tempfile.TemporaryDirectory(prefix='test-systemd-tmpfiles.')
 
 def test_line(line, *, user, returncode=EX_DATAERR, extra={}):
     args = ['--user'] if user else []
@@ -75,7 +76,7 @@ def test_uninitialized_t():
               user=True, returncode=0, extra={'env':{'HOME': os.getenv('HOME')}})
 
 def test_content(line, expected, *, user, extra={}, subpath='/arg', path_cb=None):
-    d = tempfile.TemporaryDirectory(prefix='test-systemd-tmpfiles.')
+    d = tempfile.TemporaryDirectory(prefix='test-content.', dir=temp_dir.name)
     if path_cb is not None:
         path_cb(d.name, subpath)
     arg = d.name + subpath