[E2E] Add RSpec Matcher for be_mergeable

qa/qa/page/merge_request/show.rb

@@ -311,13 +311,14 @@ def merged?
           end
         end
 
-        # Check if the MR is able to be merged
-        # Waits up 10 seconds and returns false if the MR can't be merged
-        def mergeable?
-          # The merge button is enabled via JS, but `has_element?` calls
-          # `wait_for_requests`, which should ensure the disabled/enabled
-          # state of the element is reliable
-          has_element?(:merge_button, disabled: false)
+        RSpec::Matchers.define :be_mergeable do
+          match do |page|
+            page.has_element?(:merge_button, disabled: false)
+          end
+
+          match_when_negated do |page|
+            page.has_no_element?(:merge_button, disabled: false)
+          end
         end
 
         # Waits up 10 seconds and returns false if the Revert button is not enabled