drop! t7810: turn MB_REGEX check into a lazy prereq

Cherry-pick of Patrick Steinhardt's upstream patch
<20260604-pks-t7527-fix-tap-output-v3-5-7d766ed481e4@pks.im>
("[PATCH v3 5/8] t7810: turn MB_REGEX check into a lazy prereq",
https://lore.kernel.org/git/20260604-pks-t7527-fix-tap-output-v3-5-7d766ed481e4@pks.im/).

414c2df ("t: let prove fail when parsing invalid TAP output",
2026-06-03) made `prove` reject TAP streams that contain
non-TAP lines. t7810 had been computing the `MB_REGEX` prereq
by running `LC_ALL=en_US.UTF-8 test-tool regex '^.$' '¿'` at
the top level of the script. On AlmaLinux 8 the en_US.UTF-8
locale is not installed by default, so the shell printed
"/bin/sh: warning: setlocale: LC_ALL: cannot change locale
(en_US.UTF-8)" into the script's stdout, which prove now
rejects with "Unknown TAP token".

Move the check into a `test_lazy_prereq` block. The locale
spelling is only evaluated when a test actually consults
MB_REGEX, and the prereq subprocess's stderr is collected
separately from the script's TAP stream, so the AlmaLinux 8
job no longer produces a stray TAP line.

This is committed as drop! because Patrick's series is still
under review on the mailing list; when it lands upstream, the
next merging-rebase will drop this commit naturally.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/t7810-grep.sh

@@ -18,8 +18,9 @@ test_invalid_grep_expression() {
 	'
 }
 
-LC_ALL=en_US.UTF-8 test-tool regex '^.$' '¿' &&
-  test_set_prereq MB_REGEX
+test_lazy_prereq MB_REGEX '
+	LC_ALL=en_US.UTF-8 test-tool regex "^.$" "¿"
+'
 
 cat >hello.c <<EOF
 #include <assert.h>