t1006: fix broken TAP format

When running t1006 via Meson we receive an error about invalid TAP
format:

$ meson test t1006-cat-file
1/1 t1006-cat-file OK 3.86s 420 subtests passed

stdout: 147: UNKNOWN: c308ae01840d8e620ad554ee5d77fe114dc2d912:path with spaces
stdout: 159: UNKNOWN: 3625298bf5e7c464a7d0e38ea80c2a5b5904d9a3e5b2b025b67f360e09b68dc7:path with spaces
ERROR: Unknown TAP output lines for a supported TAP version.
This is probably a bug in the test; if they are not TAP syntax, prefix them with a #

Ok: 1
Fail: 0

While Meson copes with it alright, it's still annoying to see these
errors on every test run.

The root cause of the broken format is a call to grep(1) that gets
executed outside of a test case, which has been added recently via
9fd38038b9c (t1006: update 'run_tests' to test generic object
specifiers, 2025-06-02). This call is done to determine whether a
subsequent test case is expected to succeed or fail, so it makes sense
to have it execute outside of a test case. But whenever we do that, we
must be extra careful to not generate any output that breaks the TAP
format.

Fix the issue by adding '-q' to the command so that it doesn't print
any matching lines.

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