tests/realtek: Cover the storage error path that returned a freed GError

Deleting a print whose template is not in the sensor's storage makes the
driver fail its task SSM, which is the path that handed an already-freed
GError to fpi_device_delete_complete(). Reading the reported error is
therefore what the test is for.

No enrollment is involved: the test deserializes a stored realtek print
whose template is deliberately not on the device, so the recording needs
no finger presses and the capture stays at 90 packets.

Without the previous commit the test dies rather than fails:

  umockdev-run ... died with <Signals.SIGSEGV: 11>
  1/1 drivers+custom - libfprint:realtek-storage-errors FAIL

The duplicate-enrollment path reaches the same bug through
fp_enroll_ssm_done(), but capturing it needs two full enrollments and
some 1.07M packets of polling traffic, so it is left out here.

tests/meson.build

@@ -59,6 +59,7 @@ drivers_tests = {
     'fpcmoc': {},
 	'realtek': {},
 	'realtek-5816': {},
+	'realtek-storage-errors': {},
     'focaltech_moc': { 'timeout': 30 },
     'focaltech_moc-6553': {},
     'mafpmoc': {},