Use splat operator in signature, not func_get_args

Use splat operator in signature, not func_get_args

Most of the original scope of this patch was implemented in 8665536045c

What remains is to use the splat operator in the method signature as
well which is more readable than using func_get_args and doesn't annoy
phan. This is now possible, because we are no longer testing on HHVM and
these mocking in HHVM tests was the only place where this was broken.

Change-Id: I52912d1810b955b05cd17fab424f80c201883411