Further improvements to the Fab build system in LFRic core

### Version

main

### Are there any linked Issues or Pull Requests?

No

### What happened?

While PR #246 (issue #240) is being reviewed, certain mid term improvements have been noted to further improve the Fab build system. This issue keeps track of the issues noted in the review (and will likely require individual tickets)

- [ ] Improve handling of components
- [ ] Let each app specify which component it needs
- [ ] Let each component add required preprocess flags, modules to ignore etc instead of having them all in the base class.
- [ ] Matthew wrote: "It is not possible to build the infrastructure without NetCDF but it is possible to build things without the infrastructure with the current build system. The fact that the current build system does this, does not make it right. "
- [ ] Add support to run unit-tests

### Relevant log output
N/A

Start by reading PR #246 and issue #240, then inspect the current Fab build-system entry points. The issue combines component handling and unit-test support, so split these into individually scoped tickets before implementation. Done should mean each ticket has a specific change and verification path, which this issue does not yet define.

Written by the indexing model from the issue text.