Add vadd_vv unit test

Adding `vadd.vv` with one sequential array as the test for each
SEW/LMUL pair.

Testst have been verified to pass in Qemu.

Note: current approach limits to one test_function per SEW/LMUL pair, we
may be able to increase the number of tests via adding more tests within
the test_function's macro generator.

Change-Id: I9a5fbd88173c50ed9511c07090bd8e936966aa96