fix pipelines (missing allow-plugins in composer.json)

[mringler]

I think this fixes the error currently occurring when running tests on gitlab:

 Error: dealerdirect/phpcodesniffer-composer-installer contains a Composer plugin which is blocked by your allow-plugins config. You may add it to the list if you consider it safe.
You can run "composer config --no-plugins allow-plugins.dealerdirect/phpcodesniffer-composer-installer [true|false]" to enable it (true) or disable it explicitly and suppress this exception (false)
See https://getcomposer.org/allow-plugins

In PluginManager.php line 769:

  dealerdirect/phpcodesniffer-composer-installer contains a Composer plugin w  
  hich is blocked by your allow-plugins config. You may add it to the list if  
   you consider it safe.                                                       
  You can run "composer config --no-plugins allow-plugins.dealerdirect/phpcod  
  esniffer-composer-installer [true|false]" to enable it (true) or disable it  
   explicitly and suppress this exception (false)                              
  See https://getcomposer.org/allow-plugins                                                                                        

This was confusing, since the expected statement was already present in composer.json. But the pipelines use custom composer.json files located in tests/composer/, where the statements where missing.

Let's see if this works...

[mringler]

Yup, green checks are back. Seems like the problem was on our side after all.

[dereuromark]

I wonder how those worked before so far

[mringler]

I wonder how those worked before so far

My guess is that the behavior was added with latest composer from July 13th