TST: consolidate Index.test_insert / test_delete

[jreback]

we are repeating a fair amount of code in the testing of test_insert & test_delete (repeating in the sub-class testing). This could be pulled to index/common.py and index/datetimelike.py and tested on indices (the fixture).

xref #18300

[jbrockmendel]

closing as these tests are now pretty well organized