mtest: consistently, sanely, exit "1" if tests fail

Previously, we exited as "number of test failures", as a side effect of
a refactor. Regressed in commit 297749581dd4dcb4e68da4b1d0622d2916fb8db3.

This was a bad sentinel for "track if tests have failed", and formerly
we did indeed set "returncode = 1" upon encountering a test fail! The
consequence of this is that we break:

- returning failure at all, if test fails are a multiple of 256

- returning 125 for "git bisect run shall skip this commit", as a
  documented guarantee of meson

- returning > 127 shall abort bisection entirely

The behavior is nonsensical and accidental, and nobody should be using
this accident to count the number of failed tests (as it won't even work).

Fixes: https://github.com/mesonbuild/meson/issues/15251

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