tst-getpw: Rewrite.

tst-getpw: Rewrite.

The test is rewritten to look for the testable conditions and
exit once they are all detected. This prevents the test from
iterating over 2000 UIDs and looking up each one. It speeds up
the test and prevents it from failing if the system under test
has an NSS-based passwd that is slower than the test timeout.

See:
https://sourceware.org/ml/libc-alpha/2015-01/msg00394.html