Add tests for the new cert controllers and some other small refactorings

- Add a unit test for each cert controller
- Make DynamicTLSServingCertProvider an interface and use a mutex
  internally
- Create a shared ToPEM function instead of having two very similar
  functions
- Move the ObservableWithInformerOption test helper to testutils
- Rename some variables and imports

internal/testutil/observable_with_informer_option.go

@@ -0,0 +1,30 @@
+/*
+Copyright 2020 VMware, Inc.
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package testutil
+
+import "github.com/suzerain-io/controller-go"
+
+type ObservableWithInformerOption struct {
+	InformerToFilterMap map[controller.InformerGetter]controller.Filter
+}
+
+func NewObservableWithInformerOption() *ObservableWithInformerOption {
+	return &ObservableWithInformerOption{
+		InformerToFilterMap: make(map[controller.InformerGetter]controller.Filter),
+	}
+}
+
+func (i *ObservableWithInformerOption) WithInformer(
+	getter controller.InformerGetter,
+	filter controller.Filter,
+	opt controller.InformerOption) controller.Option {
+	i.InformerToFilterMap[getter] = filter
+	return controller.WithInformer(getter, filter, opt)
+}
+
+func (i *ObservableWithInformerOption) GetFilterForInformer(getter controller.InformerGetter) controller.Filter {
+	return i.InformerToFilterMap[getter]
+}