Switch to stretchr/testify / mockery for mocks

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>

hack/tools/tools.go

@@ -31,8 +31,8 @@ import (
 	_ "github.com/cespare/prettybench"
 	_ "gotest.tools/gotestsum"
 
-	// mockgen
-	_ "go.uber.org/mock/mockgen"
+	// mockery
+	_ "github.com/vektra/mockery/v2"
 
 	// tools like cpu
 	_ "go.uber.org/automaxprocs"