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

test/test-lib.sh

@@ -136,6 +136,8 @@ add_gnupg_home ()
 
 add_gpgsm_home ()
 {
+    test_require_external_prereq openssl
+
     local fpr
     [ -e "$GNUPGHOME/gpgsm.conf" ] && return
     _gnupg_exit () { gpgconf --kill all 2>/dev/null || true; }