TST: Revisit skipped and xfailed tests

[pllim]

These are the some notes from @jhunkeler from his work on #45. Investigation is needed whether they are real bugs or just the tests need to be re-designed.

File	Test name	Note	Status
test_core_nongraphics.py	test_irafparlist_getName	BUG: Returns file path instead of name	Fixed in #76
test_core_nongraphics.py	test_irafparlist_getParList	BUG: raises 'TypeError: number coercion failed'	Fixed in #76
test_core_nongraphics.py	test_irafparlist_setParam_float	BUG: TypeError: cannot concatenate 'str' and 'float' objects	Fixed in #76
test_core_nongraphics.py	test_subproc_raise_on_impossible_execution	Child does not raise SubproccesError: It only prints the error received from a raised exception.	Fixed in #51
test_core_nongraphics.py	test_irafparlist_incompatible_assignment_raises	Can overwrite string type with uncast integer
test_graphics.py	setup_module	Does not work under Darwin, because c.OF_GRAPHICS is True
test_graphics.py	test_gki_prow_to_different_devices	Erroneously sends jobs to network printers
test_invocation.py	test_invoke_command_direct	BUG: Why is there a single newline on stderr?