tests: tell pip not to check for a newer version

pip will check to see if it's the latest version, and complain if it isn't.
The --no-index flag implies the --disable-pip-version-check flag, and makes
the warning (and any associated network activity) go away.

tests/test-install.t

@@ -188,7 +188,7 the default for them.
   $ $PYTHON -m virtualenv --no-site-packages --never-download installenv >> pip.log
 Note: we use this weird path to run pip and hg to avoid platform differences,
 since it's bin on most platforms but Scripts on Windows.
-  $ ./installenv/*/pip install $TESTDIR/.. >> pip.log
+  $ ./installenv/*/pip install --no-index $TESTDIR/.. >> pip.log
   $ ./installenv/*/hg debuginstall || cat pip.log
   checking encoding (ascii)...
   checking Python executable (*) (glob)