selftest: rename schemaupgrade_dc (+pair) to schema_dc

selftest: rename schemaupgrade_dc (+pair) to schema_dc

This is needed because the name of the autobuild job and
the name of the selftest env end up in the socket path
for ncalrpc sockets.

The challenge is that (for example)
/memdisk/autobuild/fl/b2424063/samba-schemaupgrade/bin/ab/schemaupgrade_pair_dc/ncalrpc/np/protected_storage
does not fit in a struct sockaddr_un.

Signed-off-by: Garming Sam <garming@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Fri Apr 12 05:41:36 UTC 2019 on sn-devel-144