Combine vadd_vi, vadd_vx, vadd_vv into one test

This reduces the time for running tests (as it takes around 4 seconds
to start up a new test), instead running vadd's all in one "test", but
still maintains 3 separate files, one for each instruction.

Change-Id: Ic3974f8873a7dcff2155c5e080d010cf1b59c70b