Enhancement: Apply void_return fixer to tests

[localheinz]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	n/a
Documentation	n/a
License	MIT

What's in this PR?

This PR

• applies the void_return fixer to test code

I ran

$ php-cs-fixer fix --allow-risky=yes --diff --verbose --rules=void_return tests

Why?

We require at least PHP 7.1, so why not?

Todo

• Should we also apply this fixer to production code?

[dbu]

yay, the stricter the better. i'd be in for using that on the code in src/ too, but we might need some exceptions: for everything that might bee overwritten by the user, adding : void is a BC break: https://3v4l.org/2aoJs

though the bundle should probably not have any code extension points, that would all be in the library i hope.

[localheinz]

Thank you, @dbu!

[dbu]

i merged as this is an improvement. can you look if we can set up the rule in the config file and apply it for src too or if that leads to any changes that look risky?