TST: stricter monotonicity/uniqueness tests
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Asserting a method always passes. For example ``assert df.agg`` always passes.

By making the monotonicity and uniqueness tests stricter (i.e. testing for actual True/False rather than truthy/Falsy values) I think we get better ensurance that some PR doesn't accidentally turn a property into a method or that the property doesn't accidentally returns a non-boolean return value.

Hello @topper-123! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/indexes/multi/test_monotonic.py`](https://github.com/topper-123/pandas/blob/efe37fb89ec0399e05dcb1606fcbd4be809cee3a/pandas/tests/indexes/multi/test_monotonic.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/period/test_indexing.py`](https://github.com/topper-123/pandas/blob/efe37fb89ec0399e05dcb1606fcbd4be809cee3a/pandas/tests/indexes/period/test_indexing.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/test_base.py`](https://github.com/topper-123/pandas/blob/efe37fb89ec0399e05dcb1606fcbd4be809cee3a/pandas/tests/indexes/test_base.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/test_category.py`](https://github.com/topper-123/pandas/blob/efe37fb89ec0399e05dcb1606fcbd4be809cee3a/pandas/tests/indexes/test_category.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/test_numeric.py`](https://github.com/topper-123/pandas/blob/efe37fb89ec0399e05dcb1606fcbd4be809cee3a/pandas/tests/indexes/test_numeric.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/test_range.py`](https://github.com/topper-123/pandas/blob/efe37fb89ec0399e05dcb1606fcbd4be809cee3a/pandas/tests/indexes/test_range.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_analytics.py`](https://github.com/topper-123/pandas/blob/efe37fb89ec0399e05dcb1606fcbd4be809cee3a/pandas/tests/series/test_analytics.py) !

 thanks @topper-123 