Improve testing by exporting fake.scheme

[jessequinn]

Is your feature request related to a problem? Please describe.
I would like to add corev1 schemes so that i can create a secret to test a cluster issuer. in the current state fake.scheme is not exportable; thus, i cannot add corev1 schemes.

Describe the solution you'd like
make fake.scheme fake.Scheme (exportable)

Describe alternatives you've considered
Have not thought of any.

Additional context
An example:

By("Creating a signing keypair fixture")
scheme := runtime.NewScheme()
corev1.AddToScheme(scheme)
cmfake.AddToScheme(scheme)
cmfake.Scheme = scheme
client = cmfake.NewSimpleClientset(&corev1.Secret{
	TypeMeta: metav1.TypeMeta{
		Kind:       "Secret",
		APIVersion: "v1",
	},
	ObjectMeta: metav1.ObjectMeta{
		Name: secretName,
	},
	StringData: map[string]string{
		corev1.TLSCertKey:       rootCert,
		corev1.TLSPrivateKeyKey: rootKey,
	},
})

/kind feature

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