mtest: do not warn when the TAP stream version is 12

Partial rollback of commit b7a5c384a1f1ba80c09904e7ef4f5160bdae3345. The
rationale was based on a confusing wording of the TAP14 spec, which is
under discussion for clarification / amendment.

TAP14 doesn't (shouldn't) really say that missing a version line is
potentially an error. Rather, this is the correct way to denote TAP12,
which a TAP14 harness may not understand or try to parse. The intention
was never to suggest that harnesses "should" take exception to the
missing version line on the grounds that one should really add a version
line.

So, stop emitting an annoying warning for something that's valid usage.
Meson understands TAP12 and that's okay.

However, we do need to keep the part of that commit which set the
version to 12 if it was otherwise unspecified. But instead of
distinguishing between None and a version, just default to 12.

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