tests: Run each shutil test in a temporary directory

Otherwise it could potentially race with tests in other executables that
also want to create `./test`, or interfere with other test-cases in the
same executable that expect `./test` to be nonexistent or empty.

Signed-off-by: Simon McVittie <smcv@collabora.com>

tests/test-libglnx-shutil.c

@@ -32,6 +32,7 @@
 static void
 test_mkdir_p_enoent (void)
 {
+  _GLNX_TEST_SCOPED_TEMP_DIR;
   _GLNX_TEST_DECLARE_ERROR(local_error, error);
   glnx_autofd int dfd = -1;