Tests cannot be cancelled

Bug description When executing stxtests, cancellation is not possible

Steps to reproduce the behavior Execute the following test:

resolve loop()

rules

  loop:
  loop() :- loop().

After a while, click the red cancel button in the Eclipse Progress view.

Observed behavior The test continues indefinately.

Expected behavior The test will stop executing within a few seconds, possibly with an error message.

Additional context NullCancels are passed to tests, breaking the connection with the button.

metaborg / nabl