bitmap: exclude nbits == 0 cases from bitmap test

Bitmap API handles nbits == 0 in most cases correctly, i.e. it doesn't
dereferene underlying bitmap and returns a sane value where convenient,
or implementation defined, or undef.

Implicitly testing nbits == 0 case, however, may make an impression that
this is a regular case. This is wrong. In most cases nbits == 0 is a
sign of an error on a client side. The tests should not make such an
implression.

This patch reworks the existing tests to not test nbits == 0. The
following patch adds an explicit test for it with an appropriate
precaution.

Signed-off-by: Yury Norov <ynorov@nvidia.com>