TST: refactored Excel reader tests
This PR implements more flexible Excel (spreadsheet) reader tests. The work was originally part of the larger but unsuccessful PR #9070. 

This PR includes:
- Excel reader base class tests to facilitate adding the similar tests for other readers
- not all Excel file formats (xls, xlsm, xlsx) had a corresponding test file. This PR adds the missing ones. Consequently, all Excel data files are now tested against all 3 Excel file formats.
- rename `test` to `test1` (for xls, xlsm, xlsx extensions) for consistency with `test2..4` test data files. This also means that before merging (but after Travis has ran the tests) the URL of Excel test file `test1.*` has to be changed from `davidovitch/pandas` to `pydata/pandas` in `pandas/io/tests/test_excel.py` at line [378](https://github.com/davidovitch/pandas/blob/master/pandas/io/tests/test_excel.py#L378).
