Use PG_TEST_TIMEOUT_DEFAULT in 019_replslot_limit.pl.
Oversight in
f2698ea02ca8a56f38935d2b300ac54936712558, which introduced
the variable. This lowers some 1000s timeouts to the configurable
default of 180s, due to a lack of evidence for needing the longer
timeout. The alternative was 6*PG_TEST_TIMEOUT_DEFAULT, which we can
adopt if the need arises. Given the lack of observed trouble with these
timeouts, no back-patch.