OpenSSL::Test: supported filtered command output

95-test_external_boringssl.t had a specialised run() variant to prefix
the command output so it wouldn't disturb Test::Harness.  This
functionality if now moved to the run() command, using the added
option 'prefix' that can be set to the string to prefix the output
with.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3201)