tests/frontend/logging.py: fix error message regex

Angelos Evripiotisrequested to merge
aevri/fix_logging_regex_test into master

This regex was not matching, and therefore the assert was not doing anything.

Fix the regex to match BuildStream's current output.

Add a check to make sure that we get exactly the number of expected matches. This means we get an obvious error if the regex stops matching again.

Edited by Angelos Evripiotis

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