test-histedit-edit.t: demonstrate qnew fails during a histedit (issue4366)

This was accidentally fixed by other work, but given that it's been
broken in the past, I'd like to have a test defending us against
regressions in this area, especially as we add more functionality to
histedit.

tests/test-histedit-edit.t

@@ -95,6 +95,13 commit, then edit the revision
   $ hg ci -m 'wat'
   created new head
   $ echo a > e
+
+qnew should fail while we're in the middle of the edit step
+
+  $ hg --config extensions.mq= qnew please-fail
+  abort: histedit in progress
+  (use 'hg histedit --continue' or 'hg histedit --abort')
+  [255]
   $ HGEDITOR='echo foobaz > ' hg histedit --continue 2>&1 | fixbundle
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved