Break up the linalg test suite further

This PR further breaks up the linear algebra test suite. The linalg1 test file is broken up further so that all tests starting from the least-squares routine tests are now in a new, separate linalg3 test.

The net speedup appears to be fairly minor, ~10%, but at least it was easy to do. On my local 4-core MBP, the time taken to run `make test` decreases from 5'30" to 5'04". (I don't recall linalg1 taking so long in the past...)

Ref: #5735