target/mips: Add tests for integer add MSA instruction group

These are the regression tests for integer addition MSA instruction
- various flavors of instruction add (ADD, ADDS, HADD,...).

Signed-off-by: Mateja Marjanovic <mateja.marjanovic@rt-rk.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-Id: <1551718283-4487-3-git-send-email-mateja.marjanovic@rt-rk.com>