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/README.md

@@ -0,0 +1,80 @@
+# Objx
+[![Build Status](https://travis-ci.org/stretchr/objx.svg?branch=master)](https://travis-ci.org/stretchr/objx)
+[![Go Report Card](https://goreportcard.com/badge/github.com/stretchr/objx)](https://goreportcard.com/report/github.com/stretchr/objx)
+[![Maintainability](https://api.codeclimate.com/v1/badges/1d64bc6c8474c2074f2b/maintainability)](https://codeclimate.com/github/stretchr/objx/maintainability)
+[![Test Coverage](https://api.codeclimate.com/v1/badges/1d64bc6c8474c2074f2b/test_coverage)](https://codeclimate.com/github/stretchr/objx/test_coverage)
+[![Sourcegraph](https://sourcegraph.com/github.com/stretchr/objx/-/badge.svg)](https://sourcegraph.com/github.com/stretchr/objx)
+[![GoDoc](https://godoc.org/github.com/stretchr/objx?status.svg)](https://godoc.org/github.com/stretchr/objx)
+
+Objx - Go package for dealing with maps, slices, JSON and other data.
+
+Get started:
+
+- Install Objx with [one line of code](#installation), or [update it with another](#staying-up-to-date)
+- Check out the API Documentation http://godoc.org/github.com/stretchr/objx
+
+## Overview
+Objx provides the `objx.Map` type, which is a `map[string]interface{}` that exposes a powerful `Get` method (among others) that allows you to easily and quickly get access to data within the map, without having to worry too much about type assertions, missing data, default values etc.
+
+### Pattern
+Objx uses a preditable pattern to make access data from within `map[string]interface{}` easy. Call one of the `objx.` functions to create your `objx.Map` to get going:
+
+    m, err := objx.FromJSON(json)
+
+NOTE: Any methods or functions with the `Must` prefix will panic if something goes wrong, the rest will be optimistic and try to figure things out without panicking.
+
+Use `Get` to access the value you're interested in.  You can use dot and array
+notation too:
+
+     m.Get("places[0].latlng")
+
+Once you have sought the `Value` you're interested in, you can use the `Is*` methods to determine its type.
+
+     if m.Get("code").IsStr() { // Your code... }
+
+Or you can just assume the type, and use one of the strong type methods to extract the real value:
+
+    m.Get("code").Int()
+
+If there's no value there (or if it's the wrong type) then a default value will be returned, or you can be explicit about the default value.
+
+     Get("code").Int(-1)
+
+If you're dealing with a slice of data as a value, Objx provides many useful methods for iterating, manipulating and selecting that data.  You can find out more by exploring the index below.
+
+### Reading data
+A simple example of how to use Objx:
+
+    // Use MustFromJSON to make an objx.Map from some JSON
+    m := objx.MustFromJSON(`{"name": "Mat", "age": 30}`)
+
+    // Get the details
+    name := m.Get("name").Str()
+    age := m.Get("age").Int()
+
+    // Get their nickname (or use their name if they don't have one)
+    nickname := m.Get("nickname").Str(name)
+
+### Ranging
+Since `objx.Map` is a `map[string]interface{}` you can treat it as such.  For example, to `range` the data, do what you would expect:
+
+    m := objx.MustFromJSON(json)
+    for key, value := range m {
+      // Your code...
+    }
+
+## Installation
+To install Objx, use go get:
+
+    go get github.com/stretchr/objx
+
+### Staying up to date
+To update Objx to the latest version, run:
+
+    go get -u github.com/stretchr/objx
+
+### Supported go versions
+We support the lastest three major Go versions, which are 1.10, 1.11 and 1.12 at the moment.
+
+## Contributing
+Please feel free to submit issues, fork the repository and send pull requests!