.gitignore/.distignore: ignore phpcs/phpunit config files

* `.distignore`: no need to distribute the phpcs/phpunit config files.
* `.gitignore`: Allow devs to overwrite config files for PHPUnit and PHPCS
    - The `.dist` files should be committed. However, for their personal use, devs can overrule those files with versions without `.dist`.
        Those personal versions should never be committed.

As a side-note: for PHPCS, having a personal version while still using the original `.dist` file is made very easy, as you can just import the `.dist` file as a starting point, like so:
```xml
<?xml version="1.0"?>
<ruleset name="WP-CLI">
    <rule ref="./phpcs.xml.dist"/>
</ruleset>
```

.distignore

@@ -7,6 +7,8 @@
 behat.yml
 .circleci/config.yml
 bitbucket-pipelines.yml
+phpcs.xml.dist
+phpunit.xml.dist
 bin/
 features/
 utils/

.gitignore

@@ -6,3 +6,6 @@ vendor/
 *.tar.gz
 composer.lock
 *.log
+phpunit.xml
+phpcs.xml
+.phpcs.xml