CI: Testing generated parachains in base-parachain.

[weezy20]

E2E tests involving cargo check | building a parachain (and to a lesser extent, an ink contract), can take a while on the CI runner. This slows us down for most tasks (pull requests) that do not pertain to modifying any of the current local *.templ template files which are used to customize the generated parachain.

One approach is to simply #[ignore] these tests, and have them run only on merge to main which is fine (#90). But here we discuss an alternative strategy:

pop new parachain is a result of sourcing base-parachain and applying the customization in the form of templ files that reside locally in this repo. The validation an e2e test like build_parachain makes is to effectively run pop new parachain and then run cargo build --release on it to make sure the generation was successful.

This checks two things:

1. The *.templ files are valid and up-to-date with the source template, in this case, base-parachain
2. The base-parachain builds without errors

This can be delegated to base-parachain itself, where one CI check on the base-parachain repository could download the latest pop release binary, run pop new parachain, which in turn uses the release templ files, and effectively check both the validity of the template files in pop-cli and the base-parachain itself.