Memoize migration files per migrations paths

Because the Geo tracking database uses different migration paths,
memoize the migration files per set of `paths`.

Closes gitlab-org/gitlab-ee#3701.

config/initializers/ar_speed_up_migration_checking.rb

@@ -10,7 +10,8 @@ if Rails.env.test?
         # it reads + parses `db/migrate/*` each time. Memoizing it can save 0.5
         # seconds per spec.
         def migrations(paths)
-          (@migrations ||= migrations_unmemoized(paths)).dup
+          @migrations ||= {}
+          (@migrations[paths] ||= migrations_unmemoized(paths)).dup
         end
       end
     end