tests: use [(] instead of \( to avoid warning on py3

From a recent test run of mine:

run-tests.py:1633: DeprecationWarning: invalid escape sequence '\('

This lets us encode the expectation without the badness.

Differential Revision: https://phab.mercurial-scm.org/D3621

tests/test-unified-test.t

@@ -84,7 +84,7 b' Combining esc with other markups - and h'
   foo/bar\r (no-eol) (esc)
 #endif
   $ printf 'foo/bar\rfoo/bar\r'
-  foo.bar\r \(no-eol\) (re) (esc)
+  foo.bar\r [(]no-eol[)] (re) (esc)
   foo.bar\r \(no-eol\) (re)
 
 testing hghave