test_bitmapset: Expand more the test coverage

test_bitmapset: Expand more the test coverage

This commit expands the set of tests added by 00c3d87a5cab, to bring the
coverage of bitmapset.c close to 100% by addressing a lot of corner
cases (most of these relate to word counts and reallocations).

Some of the functions of this module also have their own idea of the
result to return depending on the input values given.  These are
specific to the module, still let's add more coverage for all of them.

Some comments are made more consistent in the tests, while on it.

Author: Greg Burd <greg@burd.me>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aNR-gsGmLnMaNT5i@paquier.xyz