run-tests: allow to register any arbitrary pattern for replacement

We add a 'common-pattern.py' file that allow to define extra pattern. This seems
a cleaner approach than editing the 'run-test.py' file over and over. In
addition allowing arbitrary pattern registration will also help extension.

The format used is a python file is picked out of convenience defining a list of
tuple in 'substitutions' variable. This is picked out of convenience since it is
dead simple to implement.

The end goal is to register more pattern for Mercurial test. There are multiple
common patterns that change over time. That impact is annoying. Using pattern
emplacement for them would be handy.

The next patches will define all the needed patterns and the last patch will
mass-update the tests outputs as it was easier to do in a single pass.

tests/run-tests.py

@@ -968,6 +968,13 b' class Test(unittest.TestCase):'
             ]
         r.append((self._escapepath(self._testtmp), b'$TESTTMP'))
 
+        testdir = os.path.dirname(self.path)
+        replacementfile = os.path.join(testdir, 'common-pattern.py')
+
+        if os.path.exists(replacementfile):
+            data = {}
+            execfile(replacementfile, data)
+            r.extend(data.get('substitutions', ()))
         return r
 
     def _escapepath(self, p):

tests/test-run-tests.t

@@ -1503,3 +1503,43 b' Test cases in .t files'
   # Ran 2 tests, 0 skipped, 1 failed.
   python hash seed: * (glob)
   [1]
+
+Test automatic pattern replacement
+
+  $ cat << EOF >> common-pattern.py
+  > substitutions = [
+  >     (br'foo-(.*)\\b',
+  >      br'\$XXX=\\1\$'),
+  >     (br'bar\\n',
+  >      br'\$YYY$\\n'),
+  > ]
+  > EOF
+
+  $ cat << EOF >> test-substitution.t
+  >   $ echo foo-12
+  >   \$XXX=12$
+  >   $ echo foo-42
+  >   \$XXX=42$
+  >   $ echo bar prior
+  >   bar prior
+  >   $ echo lastbar
+  >   last\$YYY$
+  >   $ echo foo-bar foo-baz
+  > EOF
+
+  $ rt test-substitution.t
+  
+  --- $TESTTMP/anothertests/cases/test-substitution.t
+  +++ $TESTTMP/anothertests/cases/test-substitution.t.err
+  @@ -7,3 +7,4 @@
+     $ echo lastbar
+     last$YYY$
+     $ echo foo-bar foo-baz
+  +  $XXX=bar foo-baz$
+  
+  ERROR: test-substitution.t output changed
+  !
+  Failed test-substitution.t: output changed
+  # Ran 1 tests, 0 skipped, 1 failed.
+  python hash seed: * (glob)
+  [1]