tests: stop over-specifying tempfile name

Python 3 has more random characters in the default template, which is fine.

Differential Revision: https://phab.mercurial-scm.org/D2676

contrib/python3-whitelist

@@ -217,6 +217,7 b' test-merge-revert.t'
 test-merge-revert2.t
 test-merge-subrepos.t
 test-merge-symlinks.t
+test-merge-tools.t
 test-merge-types.t
 test-merge1.t
 test-merge10.t

tests/test-merge-tools.t

@@ -1558,7 +1558,7 b' Verify naming of temporary files and tha'
   $ hg update -q -C 2
   $ hg merge -y -r tip --tool echo --config merge-tools.echo.args='$base $local $other $output'
   merging f and f.txt to f.txt
-  */f~base.?????? $TESTTMP/f.txt.orig */f~other.??????.txt $TESTTMP/f.txt (glob)
+  */f~base.* $TESTTMP/f.txt.orig */f~other.*.txt $TESTTMP/f.txt (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)