This PR was squashed before being merged into the 3.1 branch (closes#19974).
Discussion
----------
Make redis host configurable in tests
| Q | A
| ------------- | ---
| Branch? | 3.1
| Bug fix? | no
| New feature? | no
| BC breaks? | no
| Deprecations? | no
| Tests pass? | yes
| Fixed tickets | -
| License | MIT
| Doc PR | -
Makes running tests on dockerized environments possible, since we can now export the `REDIS_HOST` environment variable:
```
REDIS_HOST=redis ./phpunit src/Symfony/Component/Cache
```
If this gets merged, in master we can later replace the `SYMFONY__REDIS_HOST` usage in the FrameworkBundle with the new `env(REDIS_HOST)`.
Commits
-------
c87de00 Make redis host configurable in tests
