Update tosa tools default paths and README about CTS

Make tosa_verif_* tools assume they are in the ref_model repo with a
local build directory.
Update README with profile info, and updated commands to match
assumption of local build directory.
Add pytests for model file locations util.
Fix paths for non-build items with build_sub_dirs.

Signed-off-by: Jeremy Johnson <jeremy.johnson@arm.com>
Change-Id: I70cbcb64423b6782650166507016500268a2496d