test/e2e: Minor updates to the test namespace generation function

- Update the helper function SetUpGeneratedTestNamespace -> SetupGeneratedTestNamespace.
- Update SetupGeneratedTestNamespace to only return a corev1.Namespace
  object. Call sites can access the generated namespace by indexing into
  the object's GetName() method or the metdata.Name field.
- Update SetupGeneratedTestNamespace and remove the logic that generates
  the namespace's metadata.name in the function. Call sites are
  responsible for generating a unique name now.
- Update any subscription e2e tests to use the returned corev1.Namespace
  resource's GetName() method.
- Remove the usage of the TearDown helper function in the subscription
  e2e suite. Generated namespaces (and their underlying testing
  resources) are cleaned up by deleting the namespace itself.

Signed-off-by: timflannagan <timflannagan@gmail.com>