Refactor the test `test_marking_operations`

[mwcraig]

This single test checks several things. I think this should be brokeninto separate tests with clearer names. Right it checks that:

• an empty table returned by get_markers has the correct columns.
• the is_marking attribute is not settable.
• setting the marker style actually changes the marker properties
• calling start_marking changes the properties click_center, click_drag, and scroll_pan
• calling stop_marking returns those properties back the the values they had before start_marking was called, i.e. check that property cacheing is implemented.
• no warning is issued when retrieving a marker set that does not exist
• recheck that start/stop marking caches properly
• checks that a simulated mouse click adds a marker --> this is implementation is ginga- specific <-- so it should be removed