pam_test_srv: set default value for SOFTHSM2_CONF
Currently the SOFTHSM2_CONF is not set by any fixture but some tests
sets them and other might rely on the setting done by a previous test.
This means that the tests have to run in a given order and depend on
each other.
To remove this dependency SOFTHSM2_CONF is set in the fixture to the
"default" SoftHSM2 configuration with one valid certificate. Any test
which needs a different setup must now set SOFTHSM2_CONF explicitly.
Related to https://pagure.io/SSSD/sssd/issue/3887
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>