Skip all explicitely if the number of tests is 0

Skip all explicitely if the number of tests is 0

It seems that Test::More doesn't like 0 tests, a line like this raises
an error and stops the recipe entirely:

    plan tests => 0;

So we need to check for 0 tests beforehand and skip the subtest
explicitely in that case.

Reviewed-by: Rich Salz <rsalz@openssl.org>