Linux MapView zoom tests failing in CI

[mhsmith]

This has started happening in the last 2 days:

=================================== FAILURES ===================================
__________________________________ test_zoom ___________________________________
Traceback (most recent call last):
  File "/home/runner/work/toga/toga/testbed/build/testbed/ubuntu/jammy/testbed-0.0.1/usr/lib/testbed/app/tests/widgets/test_mapview.py", line 126, in test_zoom
    assert (
AssertionError: Zoom level 9: failed 0.352 < 5.625 < 1.406
assert 5.625 < 1.406
=========================== short test summary info ============================
FAILED tests/widgets/test_mapview.py::test_zoom - AssertionError: Zoom level 9: failed 0.352 < 5.625 < 1.406
assert 5.625 < 1.406
============ 1 failed, 400 passed, 49 skipped, 5 xfailed in 59.55s =============

Example runs:

• https://github.com/beeware/toga/actions/runs/8713369803
• https://github.com/beeware/toga/actions/runs/8713338180
• https://github.com/beeware/toga/actions/runs/8697365306