tests: Avoid Meson's strict TAP parsing for now

Older versions of Meson parsed TAP output very strictly, and would fail
the test when a `g_test_message()` mentions non-UTF-8, which results
in GLib adding a prefix on stdout that is not valid TAP syntax.

Ideally our error messages should all be valid UTF-8 even if the input
is not, but that's a larger refactor that shouldn't block security fix
releases, so for now just tell Meson to get the tests' pass/fail status
from their exit status rather than parsing their stdout.

Signed-off-by: Simon McVittie <smcv@collabora.com>
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