test suite cleanup

test suite cleanup

Many of the tests were failing or putting warnings
because a valid systempreferences table is
usaully absent by the time 'make test' is run.

Fortunately, only a few modules try to invoke
C4::Context->preference during module initialization,
so added to the test suite override_context_prefs.pm,
which replaces preference() with a sub to return
testing values for three variables: 'dateformat',
'marcflavour', and 'LibraryName'.

Also fixed bug in t/Boolean.t

With this patch and the patch to move the DB-dependent
tests off to the side for the moment, 'make test'
now runs cleanly, at least on Debian.

Signed-off-by: Chris Cormack <crc@liblime.com>
Signed-off-by: Joshua Ferraro <jmf@liblime.com>