t: let prove fail when parsing invalid TAP output

t: let prove fail when parsing invalid TAP output

To make the result of our tests accessible we use the TAP protocol. This
protocol is parsed by either prove or by Meson. Unfortunately, these two
tools differ when it comes to their strictness when parsing the
protocol:

  - Prove by default happily accepts lines not specified by the
    protocol.

  - Meson will also accept such lines, but prints a big and ugly warning
    message.

We have fixed our test suite in the past to not print invalid TAP lines
anymore via b1dc2e796e (Merge branch 'ps/meson-tap-parse', 2025-06-17).
But as none of our tools perform a strict check it's still possible for
broken tests to sneak back in, like for example in 362f69547f (Merge
branch 'ps/t1006-tap-fix', 2025-07-16). This doesn't hurt at all when
using prove, but it's quite annoying when using Meson due to the
generated warnings.

Unfortunately, there doesn't seem to be a portable way to make all tools
complain about violations of the TAP format. The TAP 14 specification
has added pragmas to the protocol that would allow us to say `pragma
+strict`, and the effect of that would be to treat invalid TAP lines as
a test failure. But the release of TAP 14 is still rather recent, and
Test-Harness for example only gained support for it in version 3.48,
which was released in 2023.

In fact though, this pragma was already introduced as an inofficial
extension of the TAP protocol with Test-Harness 3.10, released in 2008.
So while not all tools understand the pragma, at least prove does for a
long time.

Unconditionally enable the pragma when using prove so that we'll detect
tests that emit broken TAP output right away. This would have detected
the issues fixed in preceding commits:

    $ prove t7527-builtin-fsmonitor.sh
    t7527-builtin-fsmonitor.sh .. All 69 subtests passed
            (less 6 skipped subtests: 63 okay)

    Test Summary Report
    -------------------
    t7527-builtin-fsmonitor.sh (Wstat: 0 Tests: 69 Failed: 0)
      Parse errors: Unknown TAP token: "Initialized empty Git repository in /tmp/git/test_fsmonitor_smoke/.git/"

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>