Clean up pg_rewind regression test script.

Since commit 01f6bb4b, 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
13d856e1, which moved the initdb-running code to TestLib.pm.

Backpatch to 9.5.