split macos tests in 2
follow up to #43468 to also split MacOS tests in half to speed up CI tests per comment from @jreback https://github.com/pandas-dev/pandas/pull/43468#issuecomment-916860226

On a run like https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=66005&view=results, the test distribution looks like windows1: `gw0 [104233] / gw1 [104233]`, windows2: `gw0 [26865] / gw1 [26865]` and windows1 seems to take generally longer (and saw a timeout yesterday, can't remember which pr). Maybe we can add a few more letters to the second job to even out? Looking at the Windows timeouts, the last lines before the timeout nearly all look like they are making HTTP requests, e.g.

```
....ss......ss......ss.x.x.xxss......ss......ss......ss................127.0.0.1 - - [10/Sep/2021 21:05:19] "GET / HTTP/1.1" 200 -
```

I could totally imagine something in the network stack blocking and not having a timeout set. > On a run like https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=66005&view=results, the test distribution looks like windows1: `gw0 [104233] / gw1 [104233]`, windows2: `gw0 [26865] / gw1 [26865]` and windows1 seems to take generally longer (and saw a timeout yesterday, can't remember which pr). Maybe we can add a few more letters to the second job to even out?

Good observation.  I moved it by one letter.  There are a lot of tests in the "i" category, so let's see how this change helps. Split now `gw0 [74211] / gw1 [74211]` on Windows1 and `gw0 [56887] / gw1 [56887]` on Windows2  thanks @Dr-Irv  Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.3.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 dfbc76ec8215437d666c00b8e6d4888fbf1c439a
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #43517: split macos tests in 2'
```

4. Push to a named branch :

```
git push YOURFORK 1.3.x:auto-backport-of-pr-43517-on-1.3.x
```

5. Create a PR against branch 1.3.x, I would have named this PR:

> "Backport PR #43517 on branch 1.3.x (split macos tests in 2)"

And apply the correct labels and milestones.

Congratulation you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove `Still Needs Manual Backport` label once the PR gets merged.

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 @meeseeksdev backport 1.3.x Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.3.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 dfbc76ec8215437d666c00b8e6d4888fbf1c439a
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #43517: split macos tests in 2'
```

4. Push to a named branch :

```
git push YOURFORK 1.3.x:auto-backport-of-pr-43517-on-1.3.x
```

5. Create a PR against branch 1.3.x, I would have named this PR:

> "Backport PR #43517 on branch 1.3.x (split macos tests in 2)"

And apply the correct labels and milestones.

Congratulation you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove `Still Needs Manual Backport` label once the PR gets merged.

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                