test: check openssl prequisite for add_gpgsm_home

test: check openssl prequisite for add_gpgsm_home

This is a fix for the test failures reported by Dan Čermák [1].

It is more robust to check for the prerequisite inside the function
that uses it, rather than in every test file that calls the function.

[1]: id:87k0n4fqgm.fsf@tethera.net