Fix ThreadActivityTest

Related to #798

Due to recent change in how exceptions are handled in `@Background`
annotated methods, the test `ThreadActivityTest.propagateException()` is
broken because the `backgroundThrowException()` doesn't throw exception
any more. This kind of methods is now covered by another test
`propagateExceptionToGlobalExceptionHandler()`.

This commit ensures also this last test cleans its custom exception
handler.