Defer tests to the morphic ui thread

[guillep]

Make Spec tests run in the UI thread to avoid concurrency issues.

The problem is more detailed in here: pharo-project/pharo#12502

Problem summary: during the test run in the CI, tests and morphic run in separate tests.
Thus, a test might change a graphic element during the morphic ui draw and break it.
If an exception happens in a thread other than the test thread, the process quits and stops the test run with an unexpected outcome.

[guillep]

There is a broken test, unrelated, and caused by some protocol recompilation.

To see the issue in isolation of this PR, the following code adds a new superclass and breaks the test:

ProperPackagesTest debug: #testPackageExtensionsStartsWithProperPackageName.

newTestCase := (TestCase << #NewTestCase)
	package: #toto;
	install.
	
(newTestCase << #SpBaseTest)
	install.
	
ProperPackagesTest debug: #testPackageExtensionsStartsWithProperPackageName.