tests/fprintd: Only enable 'virtual_image' driver

When we run tests in a system with real devices, we may try to initialize
the real ones, while we can just ignore them all in tests.

We do it in setUp instead of setUpClass to allow tests to change this if
they need to, but just for temporary.

tests/fprintd.py

@@ -262,6 +262,7 @@ class FPrintdTest(dbusmock.DBusTestCase):
         self.addCleanup(shutil.rmtree, self.test_dir)
         self.state_dir = os.path.join(self.test_dir, 'state')
         self.run_dir = os.path.join(self.test_dir, 'run')
+        os.environ['FP_DRIVERS_WHITELIST'] = 'virtual_image'
 
     # From libfprint tests
     def send_retry(self, retry_error=FPrint.DeviceRetry.TOO_SHORT):