tls/tests: disable PKCS#11 tests if not available
GnuTLS can be built without PKCS#11, which means the symbols gnutls_pkcs11_init and gnutls_pkcs11_add_provider are not part of the library.
If these symbols don't exist in GnuTLS then we can't add a mock pkcs#11 provider for testing, and several tests which need the mock provider will fail.
Solve this by checking for the symbols at build time and disabling the provider and tests which need it.