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>

vendor/github.com/stretchr/objx/security.go

@@ -0,0 +1,12 @@
+package objx
+
+import (
+	"crypto/sha1"
+	"encoding/hex"
+)
+
+// HashWithKey hashes the specified string using the security key
+func HashWithKey(data, key string) string {
+	d := sha1.Sum([]byte(data + ":" + key))
+	return hex.EncodeToString(d[:])
+}