Fix test name and username used in failed connection attempts

Fix test name and username used in failed connection attempts

The first failed connection tests the "regular" connections limit, not
the reserved limit.

In the second failed connection, the username doesn't really matter,
but since the previous successful connections used "regress_reserved",
it seems weird to switch back to "regress_regular" for the
expected-to-fail attempt.

Discussion: https://www.postgresql.org/message-id/fd5e9523-78d3-4270-86b2-fd1b1eeb4fc9@iki.fi