Separate version of flutter_quill_test and avoid publishing a new version when flutter_quill updated

[EchoEllet]

The testing utilities were mainly separated from flutter_quill so that users don't have to add flutter_test as a non-dev dependency since the testing utilities depend on it.

We have discussed this in #2276 and across other threads, in short, each package should have its own version separately, one of the reasons is compatibility though we have many other reasons, flutter_quill_test hasn't been updated since the day it's published, publishing a new version every time we make an update to flutter_quill is unnecessary, not only users and other dependencies have to update when we release major release, also it's taking more changes in commit history, duplicate more things in search result when searching in the IDE, when I search about something, I found about 5 to 7 CHANGELOGs that have the exact same change, we should either use better solution (which is possible) though I suggest that we separate them similar to how it's done in all dart and flutter packages.