Replace assertErrorRegex by standard assertRaisesRegex

[Flamefire]

We no longer need this custom function with Python 2 support being dropped.

The only remaining use case was EasyBuildError where str(e) didn't return the error as a string but as an escaped string.

See also #5009 which fixes that (single relevant commit included here to avoid test failures)

This makes it easier to use, especially for new devs being used to the unittest module and conventions for other functions like self.assertRaises.

Additionally this can be used as a context manager:

with self.assertRaises(TypeError): # similar for assertRaisesRegex
  do_this()
  do_that()