Coverage improvements of src/bin/pg_basebackup and pg_receivewal --endpos

Hi all,

The coverage of pg_basebackup is reaching 50%, which is not bad:
https://coverage.postgresql.org/src/bin/pg_basebackup/index.html
In this set pg_receivewal.c is the bad student with less than 20%.

There are a couple of causes behind that, with no tests like:
- option interactions like --create-slot and --drop-slot.
- replication slot drop.
- end of streaming.
- check handling of history files.
- looking at compressed and uncompressed WAL data.
- restart of pg_receivewal on an existing folder.
- pg_basebackup's tests don't use the progress report.

While it is possible to tackle some of those issues independently,
like pg_basebackup stuff, it seems to me that it would be helpful to
make the tests more deterministic by having an --endpos option for
pg_receivewal, similarly to what exists already in pg_recvlogical.

Thoughts?
--
Michael