Introduce test.NewTempWindow() to avoid memory leak in test windows

[Jacalz]

Description:

I noticed that we had a quite substantial memory leak in our test suite due to various tests forgetting to call .Close() on the test window that was created. This led to larger memory usage than what would be ideal. I fixed all the issues I could find and introduced a test.NewTempWindow() function to create a window that automatically closes at the end of the test.

Any places where we forgot to call Close() now uses the new function. Old uses that close the window have been left as is. Should we deprecate the old function to point people towards this safer new function?

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.

[coveralls]

coverage: 65.68% (-0.006%) from 65.686%
when pulling 7994aa1 on Jacalz:fix-test-window-memory-leak
into 759df6f on fyne-io:develop.

[andydotxyz]

I don't think we should deprecate test.NewWindow as it is a legitimate thing to do.

[andydotxyz]

Seems like a great change, but a couple of questions inline from reading it through.

[andydotxyz]

Nice improvement thanks

[Jacalz]

Thanks. Had to fix a merge conflict. Needs a re-approval, I'm afraid

[Jacalz]

Of course there were merge conflicts again. Sigh 😮‍💨