Intermittent testbed failure in Android DetailedList tests

[mhsmith]

When running CI, the testbed app sometimes raises the following failure on Android when running the tests/widgets/test_detailedlist.py::test_refresh test:

I/python.stdout: _________________________________ test_refresh _________________________________
I/python.stdout: Traceback (most recent call last):
I/python.stdout:   File "/data/data/org.beeware.toga.testbed/files/chaquopy/AssetFinder/app/tests/widgets/test_detailedlist.py", line 253, in test_refresh
I/python.stdout:     assert len(widget.data) == 101
I/python.stdout: AssertionError: assert 100 == 101
I/python.stdout:  +  where 100 = len(<toga.sources.list_source.ListSource object at 0x760b3f060b20>)
I/python.stdout:  +    where <toga.sources.list_source.ListSource object at 0x760b3f060b20> = <DetailedList:0x760b3f0bcb50>.data

[freakboy3742]

For anyone getting this error in CI - this error can be ignored; a re-run of the testbed will almost always cause the error to disappear.

It's likely caused by some odd timing in the handling of the refresh event - if the refresh hasn't fully processed, it's possible the new data generated by the refresh hasn't been fully added yet.

[freakboy3742]

I've added a potential workaround in #2103, merged prior to the release of Toga 0.4.

[freakboy3742]

Looks like the potential fix hasn't resolved the problem; it came up again in a CI run on #2218.