Adjust PL/Tcl regression test to dodge a possible bug or zone dependency.

Adjust PL/Tcl regression test to dodge a possible bug or zone dependency.

One case in the PL/Tcl tests is observed to fail on RHEL5 with a Turkish
time zone setting.  It's not clear if this is an old Tcl bug or something
odd about the zone data, but in any case that test is meant to see if the
Tcl [clock] command works at all, not what its corner-case behaviors are.
Therefore we have no need to test exactly which week a Sunday midnight is
considered to fall into.  Probe the following Tuesday instead.

Discussion: https://postgr.es/m/797.1487517822@sss.pgh.pa.us