test: also deactivate `ui.merge` when testing merge tool

We are about to introduce a default config for `ui.merge`. Since
`test-merge-tools.t` is about testing merge configuration, we need to make sure
it won't interfere.

tests/test-merge-tools.t

@@ -1,6 +1,10 b''
 test merge-tools configuration - mostly exercising filemerge.py
 
   $ unset HGMERGE # make sure HGMERGE doesn't interfere with the test
+  $ cat >> $HGRCPATH << EOF
+  > [ui]
+  > merge=
+  > EOF
   $ hg init repo
   $ cd repo