resolve: split test

Part of test-resolve.t is split into multiple tests to create an
injection point for new tests that will be added in subsequent patches.

tests/test-resolve.t

@@ -26,6 +26,13 b' failing merge'
   use 'hg resolve' to retry unresolved file merges or 'hg update -C .' to abandon
   [1]
 
+resolve -l should contain an unresolved entry
+
+  $ hg resolve -l
+  U file
+
+resolve the failure
+
   $ echo resolved > file
   $ hg resolve -m file
   $ hg commit -m 'resolved'