testify is used throughout the codebase; this switches mocks from
gomock to testify with the help of mockery for code generation.
Handlers and mocks in test/utils/oidc are moved to a new package:
mockery operates package by package, and requires packages to build
correctly; test/utils/oidc/testserver.go relies on the mocks and fails
to build when they are removed. Moving the interface and mocks to a
different package allows mockery to process that package without
having to build testserver.go.
Signed-off-by: Stephen Kitt <skitt@redhat.com>
