Clean up pg_rewind regression test script.
Since commit
01f6bb4b2, TestLib.pm has exported path to tmp_check directory,
so let's use that also for the pg_rewind test clusters etc.
Also, in master, the $tempdir_short variable has not been used since commit
13d856e17, which moved the initdb-running code to TestLib.pm.
Backpatch to 9.5.