TESTUTIL: Separate TAP output and other output by BIO filter

TESTUTIL: Separate TAP output and other output by BIO filter

Output that's supposed to be understood by a TAP parser gets its own
BIOs (|tap_out| and |tap_err|), and is only used internally within
testutils.  |bio_out| and |bio_err| is now only used for output that
shouldn't be parsed by the TAP parser, and all output written to those
BIOs are therefore always made to look like comments (it gets prefixed
with "# ").

Indentation and prefixing with "# " is reworked to use BIO_f_prefix(),
which allows us to throw away the internal BIO_f_tap().

The indentation level is now adjusted via a special function.

Fixes #12054

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12057)