Doc: make src/test/*/README match current reality.

Doc: make src/test/*/README match current reality.

Commit c3382a3c3, which moved the implementation of PG_TEST_EXTRA
from src/test/Makefile into individual test scripts, broke the
directions given in the subdirectory README files about how to run
these tests by hand.  Update.  Also mention wal_consistency_checking
in recovery/README --- that omission isn't the fault of c3382a3c3,
but it's still an omission.