UI not closed at the end of tests

[ievacerny]

Stems from a comment on a PR: #809 (comment)

There are lost of tests that don't close UI when they're done, which could lead to test interactions. It would be good to go through all the tests and fix this issue.

Example of such test:

traitsui/traitsui/tests/test_labels.py

Lines 196 to 204 in 0985824

	@skip_if_not_qt4
	def test_qt_no_labels_on_the_right_bug():
	# Bug: If one set show_left=False, show_label=False on a non-resizable
	# item like a checkbox, the Qt backend tried to set the label's size
	# policy and failed because label=None.
	with store_exceptions_on_all_threads():
	dialog = NoLabelResizeTestDialog()
	ui = dialog.edit_traits()