The build-all-plugins command unit tests currently use an actual call to flutter create, which makes them slow (especially on Windows) and non-hermetic. In addition flutter/plugins#6605 makes things inconsistent because the new code there uses our much more typical approach of mocking the process instead of actually running it, since actually running it would be even more complex.

We should really switch to mocking out the flutter create call, and just manually making a few files that look like the files we expect to modify, to make these much more "unit" unit tests rather than being more like integration tests. It would be a little less thorough in that if the actual project created by flutter create changed in a way that broke us, tests wouldn't catch it, but in that case the actual use of the command in CI would fail and we would find it that way.