Move tests that simulate web commits into test_rcs_add(), then add

Move tests that simulate web commits into test_rcs_add(), then add
a bunch more tests (that wind up exercising rcs_commit(),
rcs_commit_staged(), and rcs_recentchanges()). Extract some support
routines for brevity. Most is_in_keyword_substitution_mode() tests
are commented out because there's a bug -- non-binary files are
being added with "cvs add -kb".

Move tests that inspect recentchanges after direct CVS operations
into test_rcs_recentchanges().