Support multiple expected errors in neverallow_test.go

Extracted some common code used by visibility_test.go and
neverallow_test.go into a new function CheckErrorsAgainstExpectations
in testing.go.

Bug: 138428610
Test: m nothing
Change-Id: I6996fa52968c387e38b4a67ffa59cd07c2d524e8
Merged-In: Iafbadf12c6ffdc4d9128fcfe7f15792df5cfd020