tests: avoid false-positive in date-debug test

tests: avoid false-positive in date-debug test

When debugging an invalid date due to DST switching, the intermediate
'normalized time' should not be checked - its value can differ between
systems (e.g. glibc vs musl).

Reported by Niklas Hambüchen in
https://lists.gnu.org/r/coreutils/2019-05/msg00031.html
Analyzed by Rich Felker in
https://lists.gnu.org/r/coreutils/2019-05/msg00039.html

* tests/misc/date-debug.sh: Replace the exact normalized time
with 'XX:XX:XX' so different values would not trigger test failure.