Add test for HeapBitmapScan's broken skip_fetch optimization

Add test for HeapBitmapScan's broken skip_fetch optimization

In the previous commit HeapBitmapScan's skip_fetch optimization was removed,
due to being broken in not easily fixable ways. Add a test that verifies we
don't re-introduce this bug if somebody tries to re-add the feature.

Only add the test to master for now, it's possible it's not entirely
stable. That seems sufficient, as we're not going to re-introduce the feature
on the backbranches. I did verify that the test passes on all branches. If the
test turns out to be unproblematic, we can backpatch it later, should we feel
a need to do so.

Discussion: https://postgr.es/m/CAEze2Wg3gXXZTr6_rwC+s4-o2ZVFB5F985uUSgJTsECx6AmGcQ@mail.gmail.com