TAP implementation issue

[matthiasclasen]

Describe the bug

The TAP protocol states:
Any output line that is not a version, a plan, a test line, a diagnostic or a bail out is considered an “unknown” line. A TAP parser is required to not consider an unknown line as an error

Yet, the meson TAP parser does:
yield self.Error('unexpected input at line %d' % (lineno,))

[dcbaker]

It might make sense to have a warning here instead, but either way this should be pretty easy for someone to fix.

[bonzini]

Duplicate of #5944. Copying from there:

The part of the specification you quoted continues "... but may optionally choose to capture said line and hand it to the test harness, which may have custom behavior attached [...] TAP::Harness reports TAP syntax errors at the end of a test run".

In mtest, the TAP parser and harness are unified. It is possible to introduce an Unexpected namedtuple in the TAP parser if desired, so that we only report TAP syntax errors at the end of the test run, rather than exit as soon as they happen.