test-revert-interactive: don't escape the $ for $TESTTMP

Otherwise, sh tries to exec the literal path '${TESTTMP}/editor.sh' on Windows.
The test runs clean on OS X both with and without this change.

tests/test-revert-interactive.t

@@ -292,7 +292,7 b' 3) Do another commit on top'
   $ hg st
 
 4) Use interactive revert to recover f and change it on the fly
-  $ HGEDITOR="\"sh\" \"\${TESTTMP}/editor.sh\"" PRINTHUNK="YES" hg revert -i -r ".^^"  <<EOF
+  $ HGEDITOR="\"sh\" \"${TESTTMP}/editor.sh\"" PRINTHUNK="YES" hg revert -i -r ".^^"  <<EOF
   > y
   > e
   > EOF