Speed up TAP tests of pg_receivewal

This commit improves the speed of those tests by 25~30%, using some
simple ideas to reduce the amount of data written by pg_receivewal:
- Use a segment size of 1MB.  While reducing the amount of data zeroed
by pg_receivewal for the new segments, this improves the code coverage
with a non-default segment size.
- In the last test involving a slot's restart_lsn, generate a checkpoint
to advance the redo LSN and the WAL retained by the slot created,
reducing the number of segments that need to be archived.  This counts
for most of the gain.
- Minimize the amount of data inserted into the dummy table.

Reviewed-by: Ronan Dunklau
Discussion: https://postgr.es/m/YXqYKAdVEqmyTltK@paquier.xyz

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