MAINT: Remove self.assertFalse from testing
Title is self-explanatory.

Partially addresses #15990.
One remark for this replacement: in certain cases it might be that we want to test that something is *actually* ``False``, and not falsey (for which you would need to do `` assert xx is False`` instead of ``assert not xx``) 
But, this is difficult to say without looking in more detail into each of the tests, which is a bit too much work .. @jorisvandenbossche : Well according to `unittest` documentation, `assertFalse(expr)` checks that `bool(expr) is False`, so I think that concern might be moot. Ah, you're correct. The implementation actually just does ``if expr: raise ..``, which is equivalent. @gfyoung you're a hero for running through all these! @MaximilianR : Thanks!  Couldn't do it without bash ;) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16151?src=pr&el=h1) Report
> Merging [#16151](https://codecov.io/gh/pandas-dev/pandas/pull/16151?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3b80ed33e00957fec4a79cde19df66d809b5d3a7?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16151/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16151?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16151   +/-   ##
=======================================
  Coverage   90.83%   90.83%           
=======================================
  Files         159      159           
  Lines       50796    50796           
=======================================
  Hits        46143    46143           
  Misses       4653     4653
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.62% <ø> (ø)` | :arrow_up: |
| #single | `40.31% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16151?src=pr&el=footer). Last update [3b80ed3...ecc0590](https://codecov.io/gh/pandas-dev/pandas/pull/16151?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!