Fix GSSAPI krb5 cred ccache import

json_to_ccache was incorrectly indexing the JSON array when restoring
a memory ccache.  Fix it.

Add test coverage for a multi-cred ccache by exporting/importing the
synthesized S4U2Proxy delegated cred in t_s4u2proxy_krb5.c; move
export_import_cred from t_export_cred.c to common.c to facilitate
this.  Make a note in t_export_cred.py that this case is covered in
t_s4u.py.

ticket: 7706
target_version: 1.11.4