parametrize tests, unify repeated tests
There's more to do, but the diff is already pretty big as it is.

There are a handful of test methods that are defined in 5-6 test classes.  By moving those to the base class we a) get rid of duplicate code and b) run the tests for all (well, most) `DateOffset` subclasses.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21405?src=pr&el=h1) Report
> Merging [#21405](https://codecov.io/gh/pandas-dev/pandas/pull/21405?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ab668b0a56a9f2aee959bde787e9a0af4068d7a7?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21405/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21405?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21405      +/-   ##
==========================================
+ Coverage   91.89%    91.9%   +<.01%     
==========================================
  Files         153      153              
  Lines       49600    49600              
==========================================
+ Hits        45580    45583       +3     
+ Misses       4020     4017       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.89% <ø> (+0.03%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21405?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/21405/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.24% <0%> (+0.24%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21405?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/21405?src=pr&el=footer). Last update [ab668b0...d378120](https://codecov.io/gh/pandas-dev/pandas/pull/21405?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 