pgsql: 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: /messages/by-id/CAEze2Wg3gXXZTr6_rwC+s4-o2ZVFB5F985uUSgJTsECx6AmGcQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/24da5b239a4b50798fb2cbe46597cc932a5210f2

Modified Files
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.../isolation/expected/index-only-bitmapscan.out | 48 +++++++++
src/test/isolation/isolation_schedule | 1 +
.../isolation/specs/index-only-bitmapscan.spec | 108 +++++++++++++++++++++
3 files changed, 157 insertions(+)