python,misc: Fix develop resources URL to v21-0

This was incorrectly kept as `http://dist.gem5.org/dist/develop` in the
v21.0.0 release of gem5. The `dist/develop` directory is used by the
develop branch, not by gem5 releases. This change updates the URL to
point towards the currect v21-0 branch, which will remain stable and
contain resoruces always compatible with the v21-0 release.

Change-Id: I5d9a9497cebffa91f08be253f1637e11e0d5e62c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44725
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/ext/testlib/configuration.py b/ext/testlib/configuration.py
index 1fffab4..18bebdd 100644
--- a/ext/testlib/configuration.py
+++ b/ext/testlib/configuration.py
@@ -213,7 +213,7 @@
                                                       os.pardir,
                                                       os.pardir))
     defaults.result_path = os.path.join(os.getcwd(), 'testing-results')
-    defaults.resource_url = 'http://dist.gem5.org/dist/develop'
+    defaults.resource_url = 'http://dist.gem5.org/dist/v21-0'
     defaults.resource_path = os.path.abspath(os.path.join(defaults.base_dir,
                                             'tests',
                                             'gem5',