Teach generate_expected_selftests.py to null-out line numbers in .tap files

We've recently added a new output format, *.tap files, that uses formats unfamiliar to generate_expected_selftests.py, which consequently doesn't replace all line numbers with 0.
Although the selftest knows to ignore differences in line numbers, the generated output that we save as expected_*.tap gets line number variations, causing noise (and potential conflicts) during merges and when we update the files due to real changes.