TST/CLN: declass smaller test files in tests\io\excel
xref https://github.com/pandas-dev/pandas/pull/26755#pullrequestreview-247416090

the larger files `pandas/tests/io/excel/test_readers.py` and `pandas/tests/io/excel/test_writers.py` have been left for separate PRs

any idiom changes other than required to declass are also omitted to make the diff easier to check.