TST: Fix issues with inconsistent tests

[jbellister-slac]

Context

This is an attempt to fix #1012.

For the first half of that issue (test_line_edit.py), this adds retries to the change in focus of the line edit while waiting for qtbot to confirm that the change actually took place. This appears to have made these stable for the tests I already changed in this way, and this doesn't affect what they are actually testing.

For the stylesheet test failues in the issue, the only test that fails in isolation when a custom stylesheet is set is this one:

FAILED pydm/tests/utilities/test_stylesheet.py::test_stylesheet_get_style_data - assert 2 == 1.

The others are fine when run by themselves even with a stylesheet set.

But test_stylesheet_get_style_data makes changes to the PYDM_STYLESHEET environment variable, as well as the locally stored stylesheet, and does not clean up after itself if it fails. Then those left over modifications cause the other tests later in the run to fail too.

So the first fix is just to get that test to pass when a custom stylesheet is set. Then I also added a fixture to make sure it always cleans up after even if it fails, and set both tests in that file to use it since the other one has the same problem too.

Testing

Ran the tests with PYDM_STYLESHEET set to https://github.com/pcdshub/vacuumscreens/blob/master/styleSheet/masterStyleSheet.qss

[nstelter-slac]

nice! just 1 minor spelling issue.

like the use of 'yield' in 'save_and_restore_pydm_stylesheet', keeps code clean.

[YektaY]

Looks good to me