test-parse-date: defines explicit start/end dates for DST

Prior to this patch, DST times where tested by specifying a custom TZ
environment variable that didn't defined DST transition times.

Due to a bug in glibc, the test fail on 32bits platforms that use timezone
files generated by zic from tzcode >= 2014c (glibc >= 2.20).

See https://sourceware.org/bugzilla/show_bug.cgi?id=19738

By defining explicit transition times for DST in the TZ environment variable,
the test is now independant to how the system guess those transition times.

tests/test-parse-date.t

@@ -28,10 +28,12 b' This runs with TZ="GMT"'
 
 Check with local timezone other than GMT and with DST
 
-  $ TZ="PST+8PDT"
+  $ TZ="PST+8PDT+7,M4.1.0/02:00:00,M10.5.0/02:00:00"
   $ export TZ
 
 PST=UTC-8 / PDT=UTC-7
+Summer time begins on April's first Sunday at 2:00am,
+and ends on October's last Sunday at 2:00am.
 
   $ hg debugrebuildstate
   $ echo "a" > a