TAP version 14 introduced subtests, that are supposedly backward compatible
because "TAP13 specifies that non-TAP output should be ignored". Meson
reported TAP syntax errors based on behavior of "prove" at the time,
but it seems that now "prove" has become a lot more lenient; it even
accepts the following completely bogus input just fine:
---
ok 1
ok 2
x
1..1
---
So do the same and make Meson's parser accept invalid TAP input silently.
Fixes: #10032