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/tests.go

@@ -0,0 +1,17 @@
+package objx
+
+// Has gets whether there is something at the specified selector
+// or not.
+//
+// If m is nil, Has will always return false.
+func (m Map) Has(selector string) bool {
+	if m == nil {
+		return false
+	}
+	return !m.Get(selector).IsNil()
+}
+
+// IsNil gets whether the data is nil or not.
+func (v *Value) IsNil() bool {
+	return v == nil || v.data == nil
+}