tests: Consistently use pg_basebackup -cfast --no-sync to accelerate tests.

tests: Consistently use pg_basebackup -cfast --no-sync to accelerate tests.

Most tests invoking pg_basebackup themselves did not yet use -cfast, which
makes pg_basebackup take considerably longer. The only reason this didn't
cause the tests to take many minutes is that spread checkpoints only throttle
when writing out a buffer and there aren't that many dirty buffers in the
tests...

Discussion: https://postgr.es/m/20220117195711.xx4qbxutrrlmo2dg@alap3.anarazel.de