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/.codeclimate.yml

@@ -0,0 +1,21 @@
+engines:
+  gofmt:
+    enabled: true
+  golint:
+    enabled: true
+  govet:
+    enabled: true
+
+exclude_patterns:
+- ".github/"
+- "vendor/"
+- "codegen/"
+- "*.yml"
+- ".*.yml"
+- "*.md"
+- "Gopkg.*"
+- "doc.go"
+- "type_specific_codegen_test.go"
+- "type_specific_codegen.go"
+- ".gitignore"
+- "LICENSE"