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.

(cherry picked from commit 8e47e8bc)
Bug: 138428610
Test: m nothing
Change-Id: Iafbadf12c6ffdc4d9128fcfe7f15792df5cfd020