Tests: Update DeePKS UT to make sure it covers some checks.

[ErjieWu]

Unit Tests and/or Case Tests for my changes

• Unit test (multi-k) for DeePKS is changed to make sure it covers the check mentioned in issue Result of vdrpre maybe wrong in DeePKS. #6474. Cell is set to smaller one to make sure there are overlaps between l>0 orbitals in different Bravo lattice. This check will inevitably make the reference file larger.
• Use smaller projected orbital for DeePKS UT to reduce the size of reference files.

What's changed?

• Add a index check for vdr in DeePKS to make sure the index will not be out of range we want.
• Add a WARNING_QUIT if model file not exist in DeePKS.

[ErjieWu]

@mohanchen Simplification for UT finished. Ready for merge.