test: simplify user ID handling

The user ID on the self-test is a little bit clunky-looking.  It also
may end up showing up elsewhere in the test suite.  Centralizing the
user ID in one place should make it easier to handle if it ever
changes, and should make tests easier to read.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>