test/recipes/80-test_cmp_http.t: Simplify test_cmp_http()

test/recipes/80-test_cmp_http.t: Simplify test_cmp_http()

test_cmp_http() made some assumptions about what values that exit_checker
could get that aren't quite right.

Furthermore, the expected result isn't about exit codes, but about
true or false.  This is better served by getting the value from
OpenSSL::Test::run(), and checking that value against $expected_result
with Test::More::is().

Fixes #15557
Fixes #15571

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/15580)