Clean Up Testing of Excel Engine Option Setting for Readers/Writers
Follow up to this https://github.com/pandas-dev/pandas/pull/25092#issuecomment-506752805

Right now for readers we test directly setting `engine=` as a keyword argument, whereas in the writer tests we set the global option. I *think* we should consistently set the keyword argument and simply have a test on both reading / writing that setting the option will actually trigger that keyword argument to improve coverage and keep things simple