use compile_isolated for tests
Improved class comment for TestLoopTypes (test_ufuncs.py)
tweak on unittets

- always run all the loops, report all failed ones on failure
- changed an ‘assertFalse’ on a known True to use the method ‘fail’
improved/corrected on doctoring for supported_letter_types.
fixed a typo
fix the check for valid output types
simplified a weird ‘max’
improved doctoring of _dict_of_kernels_function
improved error messages, cleaned up checks for supported loop