t5326: move tests to t/lib-bitmap.sh

t5326: move tests to t/lib-bitmap.sh

In t5326, we have a handful of tests that we would like to run twice:
once using the MIDX's new `RIDX` chunk as the source of the
reverse-index cache, and once using the separate `.rev` file.

But because these tests mutate the state of the underlying repository,
and then make assumptions about those mutations occurring in a certain
sequence, simply running the tests twice in the same repository is
awkward.

Instead, extract the core of interesting tests into t/lib-bitmap.sh to
prepare for them to be run twice, each in a separate test script. This
means that they can each operate on a separate repository, removing any
concerns about mutating state.

For now, this patch is a strict cut-and-paste of some tests from t5326.
The tests which did not move are not interesting with respect to the
source of their reverse index data.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>