tests: explicitly flush output streams

Some tests fail while running with chg because they do not flush their output
streams. chgserver will make sure ui.flush is called after dispatch, but not
after {ui,repo}setup. For other non-ui streams, it should be explicitly
flushed since the request handler will use os._exit.
This patch adds explicit flushes in test-bundle2-format.t, test-extension.t
and test-obsolete.t. It will fix most test cases of them when running with chg.

tests/test-bundle2-format.t

@@ -166,6 +166,8 b' Create an extension to test bundle2 API'
   >             file.write(chunk)
   >     except RuntimeError, exc:
   >         raise error.Abort(exc)
+  >     finally:
+  >         file.flush()
   > 
   > @command('unbundle2', [], '')
   > def cmdunbundle2(ui, repo, replypath=None):
@@ -194,9 +196,9 b' Create an extension to test bundle2 API'
   >     for rec in op.records['changegroup']:
   >         ui.write('addchangegroup return: %i\n' % rec['return'])
   >     if op.reply is not None and replypath is not None:
-  >         file = open(replypath, 'wb')
-  >         for chunk in op.reply.getchunks():
-  >             file.write(chunk)
+  >         with open(replypath, 'wb') as file:
+  >             for chunk in op.reply.getchunks():
+  >                 file.write(chunk)
   > 
   > @command('statbundle2', [], '')
   > def cmdstatbundle2(ui, repo):

tests/test-extension.t

@@ -7,9 +7,11 b' Test basic extension support'
   > command = cmdutil.command(cmdtable)
   > def uisetup(ui):
   >     ui.write("uisetup called\\n")
+  >     ui.flush()
   > def reposetup(ui, repo):
   >     ui.write("reposetup called for %s\\n" % os.path.basename(repo.root))
   >     ui.write("ui %s= repo.ui\\n" % (ui == repo.ui and "=" or "!"))
+  >     ui.flush()
   > @command('foo', [], 'hg foo')
   > def foo(ui, *args, **kwargs):
   >     ui.write("Foo\\n")
@@ -183,6 +185,7 b' Check absolute/relative import of extens'
   > import foo
   > def extsetup(ui):
   >     ui.write('(extroot) ', foo.func(), '\n')
+  >     ui.flush()
   > EOF
 
   $ cat > $TESTTMP/extroot/foo.py <<EOF
@@ -1077,6 +1080,7 b' Commands handling multiple repositories '
   > from mercurial import extensions
   > def reposetup(ui, repo):
   >     ui.write('reposetup() for %s\n' % (repo.root))
+  >     ui.flush()
   > EOF
   $ hg init src
   $ echo a > src/a

tests/test-obsolete.t

@@ -935,7 +935,7 b' Test heads computation on pending index '
   >   opts['message'] = 'Test'
   >   opts['logfile'] = None
   >   cmdutil.amend(ui, repo, commitfunc, repo['.'], {}, pats, opts)
-  >   print repo.changelog.headrevs()
+  >   ui.write('%s\n' % repo.changelog.headrevs())
   > EOF
   $ cat >> $HGRCPATH << EOF
   > [extensions]