CLN: tests for str.cat
Cleaning up some left-overs from #20347 and preparing the tests for changing the default of `join` to `'left'` in 1.0 (which would break some tests that assume no alignment currently). As a side benefit, this has:
* nicer parametrization
* nicer switches (and reduced usage of them within the tests)
* now follows `result / expected` pattern instead of calculating result within `assert_..._equal`
@h-vetinari : Anaconda servers are experiencing some `503` issues.  I'll restart tests later. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22575?src=pr&el=h1) Report
> Merging [#22575](https://codecov.io/gh/pandas-dev/pandas/pull/22575?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e2e1a1051576a48f210ce17272fc24b90ebcf24a?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/22575/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22575?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22575   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files         169      169           
  Lines       50782    50782           
=======================================
  Hits        46742    46742           
  Misses       4040     4040
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <ø> (ø)` | :arrow_up: |
| #single | `42.3% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22575?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22575?src=pr&el=footer). Last update [e2e1a10...2e1a3b6](https://codecov.io/gh/pandas-dev/pandas/pull/22575?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @h-vetinari! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/tests/test_strings.py`](https://github.com/h-vetinari/pandas/blob/54721b988c1ec6c718d943eaae7f1f9037fd3ed9/pandas/tests/test_strings.py) !



##### Comment last updated on September 04, 2018 at 23:02 Hours UTC @jreback 
> pls don't use abbreviations, eg. always use expected

This module's full of `exp` (maybe even the majority), but I replaced it in the diff I'm working on now. not sure what's up with these `TestExcelWriter` test failures...? rebasing didn't help.

Edit: saw that 22601 solved it. Rebased again. @jreback All green thanks @h-vetinari 