Add `fix` command to `runTests.sh`

[oliverklee]

Goal

As a developer, I would like to have a single command in runTests.sh to run all fixers (similar to composer fix, but without using the composer scripts).

Acceptance Criteria

• We have a command runTests.sh -s fix that runs all fixers that composer fix runs.
• This is documented in the runTests.sh help.
• This command also is checked in the check-fixers CI job.
• This command does not run composer fix.