Add test.NewTempApp() to tear down app at the end of the test

[Jacalz]

Description:

Similar to #4849 but more about convenience than memory leaks.

There are also a few cases where we did not create a new app but only created a new one afterwards.
This PR should get us one step close towards being able to run tests in random order and hopefully get rid of strange order constraints when running the tests.

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: 65.091% (-0.004%) from 65.095%
when pulling 48034e0 on Jacalz:temp-test-app
into 4fa888a on fyne-io:develop.