Better test handling and logging in grp_pwd_test.cpp

grp_pwd_test.cpp uses ASSERT_ where EXPECT_ is possible, causing
missing failures when multiple failures are present.

Also, print out the uid/gid in the _iterate tests, to make debugging
easier.

Test: these same unit tests
Change-Id: I839b74fb86232b3e896091e76ab862cc5b6b8782