tests/tcg: target/mips: Add utility function reset_msa_registers()

tests/tcg: target/mips: Add utility function reset_msa_registers()

Add function reset_msa_registers() and utilize it in each MSA test.
This is needed to ensure independency of test results on the state of
MSA registers before test execution. This also allows for correction
of tests for VSHF* instructions, that are now independent on the
previous state of MSA registers.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1559838440-9866-9-git-send-email-aleksandar.markovic@rt-rk.com>