Fix intermittent test failure in 80-test_cmp_http.t

Fix intermittent test failure in 80-test_cmp_http.t

output to stderr is unbuffered bypassing the normal output, which does
not happen at line boundaries and is therefore confusing the TAP parser.
This is known to cause random test failures like this one:

80-test_cmp_http.t                    (Wstat: 0 Tests: 5 Failed: 0)
  Parse errors: Tests out of sequence.  Found (6) but expected (5)
                Bad plan.  You planned 6 tests but ran 5.

Fixes #23992

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26363)