* Make use of `prefix` in the Cray manifest schema (prepend it to
the relative CC etc.) - this was a Spack error.
* Warn people when wrong-looking compilers are found in the manifest
(i.e. non-existent CC path).
* Bypass compilers that we fail to add (don't allow a single bad
compiler to terminate the entire read-cray-manifest action).
* Refactor Cray manifest tests: module-level variables have been
replaced with fixtures, specifically using the `test_platform`
fixture, which allows the unit tests to run with the new
concretizer.
* Add unit test to check case where adding a compiler raises an
exception (check that this doesn't prevent processing the
rest of the manifest).
3969653f1b