Refactor linear algebra tests for special matrices

Tests for special matrices now live in test/linalg4.jl (formerly part of test/linalg2.jl), continuing the refactoring process for linear algebra tests (#5735).

Added tests for binary operations over special matrix types (to catch errors like #7073).

Also cleans up the Makefile entry so that only the "linalg" test is specified (it is automatically aliased to "linalg[1-4]" in runtests.jl).