test-extension: do not depend on demandimport (issue5012)

When demandimport is disabled, the test will fail because extroot/foo.py uses
import outside PYTHONPATH. This is bad since we have a PyPy migration plan and
it does not support demandimport. Fix by setting PYTHONPATH.

tests/test-extension.t

@@ -206,7 +206,7 b' Check absolute/relative import of extens'
   > from extroot.bar import s
   > buf.append('from extroot.bar import s: %s' % s)
   > EOF
-  $ hg --config extensions.extroot=$TESTTMP/extroot root
+  $ (PYTHONPATH=${PYTHONPATH}${PATHSEP}${TESTTMP}; hg --config extensions.extroot=$TESTTMP/extroot root)
   (extroot) from extroot.bar import *: this is extroot.bar
   (extroot) import extroot.sub1.baz: this is extroot.sub1.baz
   (extroot) import extroot: this is extroot.__init__