pgsql: Further refinement of stuck_on_old_timeline recovery test

Further refinement of stuck_on_old_timeline recovery test

TestLib::perl2host can take a file argument as well as a directory
argument, so that code becomes substantially simpler. Also add comments
on why we're using forward slashes, and why we're setting
PERL_BADLANG=0.

Discussion: https://postgr.es/m/e9947bcd-20ee-027c-f0fe-01f736b7e345@dunslane.net

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e46f6a078c1a4a2293839c9c5560517b2c4e1d12

Modified Files
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src/test/recovery/t/025_stuck_on_old_timeline.pl | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)