[yaxt] Preferred version is a branch

[dominichofer]

The preferred version of yaxt is a branch. https://github.com/C2SM/spack-c2sm/blob/main/repos/c2sm/packages/yaxt/package.py#L13
Thus our tests may fail due to that branch receiving commits. If the tests fail, it's okay to delete them, since we decided to do so with branch based tests.
This issue should be tackled by the future maintainer of yaxt.