TST: Fix indexes test inheritance / fixturization
Closes https://github.com/pandas-dev/pandas/issues/23746

Before:

```
7355 passed, 852 skipped, 5 xfailed, 13 warnings in 75.20 seconds
```

After:

```
5740 passed, 724 skipped, 5 xfailed, 12 warnings in 67.73 seconds 
```