Changes to update_test_data testing mode

[simonbray]

I tried to improve the 'update test data' mode to make it a bit more user-friendly, see #1070. Ping @fubar2.

The currently behaviour is that the --update_test_data flag just causes the job_output_files dir to be copied back to the test dir. So the test results confusingly do not actually relate to the new updated data at all. Furthermore, if tests fail, the command needs to be rerun to check that they work with the new data.

New behaviour in this PR when the --update_test_data flag is used:

• if tests fail, tests are automatically rerun after the test data has been updated, so that the results the user gets at the end relate to the new data, as expected. The user is informed that Test data were updated and tests were rerun.
• if the tests pass, the test data are not updated and the user is explicitly informed that All x test(s) executed passed. Therefore, no test data were updated.

[bgruening]

👍 I guess this will also be useful for automatic update PRs.

[fubar2]

Nice one, thanks @simonbray - that seems like a logical fix to me - will take a look shortly.