Merging the travis scripts will need some updates on several travis.yml files. Will do that next week

I've updated some more parts of the code and all tests seem to run on PHP 7.4 now.
Unfortunately it seems the version of imagemagick preinstalled on bionic distribution has problems with comparing the very big images. Those compares currently result in a

the 'compare' command output could not be parsed, should be an integer, got: compare: 
../../magick/quantum.c:216: DestroyQuantumInfo: Assertion `quantum_info != (QuantumInfo *) NULL' failed.

Seems the policies set for imagemagick differs. While the limitations for image size or memory usage are commented out on trusty builds, they are limited on bionic builds. Maybe that causes that error. Will have a closer look at that tomorrow.

Is this a travis issue maybe? Could create a bug report there and wait for a month or two to see if something happens? Running tests on PHP 7.4 might not be highest priority so it could wait?

I suppose we can't use the old distro for UI tests and run it on older PHP versions easily?

tried using an own policy for imagemagick, but that doesn't work either. Will check if we can use xenial for the UI build and if the error is the same there...