TST: re-enable tests on table formats, update them, test for "center" argument to as_latex_tabular"

The tests on as_latex_tabular(), as_text(), and as_html() were apparently disabled (defined as nested functions) since their introduction, in 5ce7d818f401b720264ce592dcd0380324d4c491.
They are now ran from inside test_SimpleTable_4, of which they use the definition of "tbl".