pgsql: Retry initial slurp_file("current_logfiles"), in test 004_logrot

Retry initial slurp_file("current_logfiles"), in test 004_logrotate.pl.

Buildfarm member topminnow failed when the test script attempted this
before the syslogger would have created the file.  Back-patch to v12,
which introduced the test.

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9a02dbdd03e8eadbe5331311c122548fac156f56

Modified Files
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src/bin/pg_ctl/t/004_logrotate.pl | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)