Add e2e test for namespace checker filter

[PrasadG193]

Is your feature request related to a problem? Please describe.

Add an e2e test to verify namespace checker filter https://github.com/infracloudio/botkube/blob/develop/test/e2e/filters/filters.go#L44

Additional context

This is a followup task of https://github.com/infracloudio/botkube/pull/437