Bug 16500: [QA Follow-up] Catch warning if branch MPL already exists

Bug 16500: [QA Follow-up] Catch warning if branch MPL already exists

The current test had no warnings, when you have no MPL branch, and only
catches the warn from the second build on line 57. If you do have MPL,
a warning comes from line 56.

This patch fixes it by removing MPL from the test. A new branch is created
first and then we try to create it again.

Test plan:
Run the test.

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