Behat: Add rename co-author tests

Not comprehensive testing, but they do currently pass.

features/rename-coauthor.feature

@@ -0,0 +1,45 @@
+Feature: Co-authors can be renamed
+
+	Background:
+		Given a WP installation with the Co-Authors Plus plugin
+
+	Scenario: Error on a missing required --from parameter
+		When I try `wp co-authors-plus rename-coauthor --to="still-not-a-user"`
+		Then STDERR should be:
+          """
+	      Error: Parameter errors:
+           missing --from parameter
+          """
+
+	Scenario: Error on a missing required --to parameter
+		When I try `wp co-authors-plus rename-coauthor --from="not-a-user"`
+		Then STDERR should be:
+          """
+	      Error: Parameter errors:
+           missing --to parameter
+          """
+
+	Scenario: Error on an invalid co-author
+		When I try `wp co-authors-plus rename-coauthor --from="not-a-user" --to="still-not-a-user"`
+		Then STDERR should be:
+          """
+	      Error: No co-author found for not-a-user
+          """
+
+	Scenario: Try to rename co-author to the same name
+		When I try `wp co-authors-plus create-guest-authors`
+		Then I try `wp co-authors-plus rename-coauthor --from="admin" --to="admin"`
+		Then STDERR should be:
+          """
+	      Error: New user_login value conflicts with existing co-author
+          """
+
+	Scenario: Rename co-author
+		When I run `wp co-authors-plus create-guest-authors`
+		Then I run `wp co-authors-plus rename-coauthor --from="admin" --to="renamed-admin"`
+		Then STDOUT should be:
+          """
+	      Renaming admin to renamed-admin
+	      Updated guest author profile value too
+	      Success: All done!
+          """