testutil: Fix non-standard subtest output

In some cases, testutil outputs subtests like this:

    1..6 # Subtest: progname

The standard set by Test::More (because there really is no actual
standard yet) gives this display:

    # Subtest: progname
    1..6

Until the standard is actually agreed upon, let's do it like
Test::More.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3446)