Where is SSPI auth username determined for TAP tests?

bowerbird is failing the pg_dump regression tests with a lot of

FATAL:  SSPI authentication failed for user "regress_postgres"

I think this is likely a consequence of ca129e58c0 having modified
010_dump_connstr.pl to use "regress_postgres" not "postgres" as the
bootstrap superuser name in the source cluster.  I suppose I overlooked
some dependency on the user name that only affects SSPI ... but what?
I don't see anything about the destination cluster configuration (which
already used a nondefault superuser name) that I didn't replicate
in the source cluster configuration.

			regards, tom lane