test: Run buildpack tests separately

Two reasons:

* Tests can be run individually when debugging specific buildpack issues
* On failure, easier to detect which buildpack(s) failed

I have also merged `BasicSuite` and `BuildpackSuite` into `GitDeploySuite` to remove some duplication. I was also tempted to remove `TestBasic` as it it almost identical to `TestNodejsBuildpack`, any reason to keep it?
Oops, missed the return.