Bug 18996: [QA Follow-up] Remove global variables from Message.t

Bug 18996: [QA Follow-up] Remove global variables from Message.t

As per request of Colin in comment18, this patch makes the use of global
variables in Message.t no longer needed.
The three subtests are now completely independent and could well be moved
to separate test scripts.

Note: Strictly speaking, the use of global (package) variables could
potentially introduce new bugs (e.g. if the value is modified outside the
script). This seems not to be the case here, but we are safe now.

Test plan:
Run the test again.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Colin Campbell <colin.campbell@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>