tests: fix SSL test with glib-networking >= 2.65.90
To make SSL tests fail with our testing certificate we create and empty GTlsDatabase passing /dev/null to g_tls_file_database_new(). This no longer works with newer glib-networking, since an empty file is considered an error by gnutls and g_tls_file_database_gnutls_populate_trust_list() now handles gnutls errors properly. Instead, we can just use the system CA file that won't contain our testing certificate for sure.
Bug: https://gitlab.gnome.org/GNOME/libsoup/-/issues/201 Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=970935 Applied-upstream: 2.72.1, commit:70cdb390e43c40970de38d8b7e75d6296a6e2a67
Enable the tests in d/rules.