pgsql: Clean up pg_rewind regression test script.

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.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2b917a58aec17ca5cf64196ee1d5d77ef8635caf

Modified Files
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src/bin/pg_rewind/RewindTest.pm |   21 ++++++---------------
1 file changed, 6 insertions(+), 15 deletions(-)