Fix flake in manifest_diff_displayer_test
Fix flake in manifest_diff_displayer_test

In a certain random ordering, the "it does not return an error" test
here would fail because the manifest had been set up to be invalid by a
different test. With this change, making sure to initialize rawManifest
to an empty byte array in a top-level BeforeEach to prevent test
pollution.

cloudfoundry/cli,cloudfoundry/cli,cloudfoundry/cli