[IMP] attempt (again) to fix testing of datetime pattern conversions tests

In order to ensure that the conversion is at least sensible, we test
the output of strftime and babel on the same pattern (after conversion
from POSIX to LDML for Babel). This requires that both use the same
locale.

Previously, the default locale was being used(-ish), but it turns out
when the locale is unset (getdefaultlocale() returns (None, None)
``strftime`` still manages to use some sort of default locale (C?),
whereas Babel will yield the locale None. It turns out the runbot
executes the server with no locale-related envvar set, and thus hits
this very issue.

Assume there are no concurrent tests being run and use setlocale to
try and ensure strftime and babel see the same locale.

Also, remove tests on %x and %X, turns out libc and babel generally
have very different ideas about what constitutes a "national
representation" of a date or time, so the patterns don't match in the
first place. And I found no way to expand %x/%X into its sub-patterns.