test-http-bad-server: move the extension in `testlib`

This seems like a better location for it.

Differential Revision: https://phab.mercurial-scm.org/D12036

tests/test-check-code.t

@@ -33,7 +33,7 b' New errors are not allowed. Warnings are'
   Skipping contrib/packaging/hgpackaging/wix.py it has no-che?k-code (glob)
   Skipping i18n/polib.py it has no-che?k-code (glob)
   Skipping mercurial/statprof.py it has no-che?k-code (glob)
-  Skipping tests/badserverext.py it has no-che?k-code (glob)
+  Skipping tests/testlib/badserverext.py it has no-che?k-code (glob)
 
 @commands in debugcommands.py should be in alphabetical order.
 

tests/test-http-bad-server.t

@@ -30,7 +30,7 b' and because debugging partial responses '
 
   $ cat > .hg/hgrc << EOF
   > [extensions]
-  > badserver = $TESTDIR/badserverext.py
+  > badserver = $TESTDIR/testlib/badserverext.py
   > [server]
   > compressionengines = none
   > EOF