[PATCH 03 of 19] tests: add a `tests/artifacts/` directory

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1534589144 -7200
#      Sat Aug 18 12:45:44 2018 +0200
# Node ID a4f94a5caf6f652ff4959daedddf1d88c6059f1b
# Parent  2d50819efa01dbb1ba0e14e30486490cb90c002a
# EXP-Topic sparse-snapshot
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r a4f94a5caf6f
tests: add a `tests/artifacts/` directory

That directory is meant to cache large items used by tests that are slow to
generate. See 'PURPOSE' file for details and next changesets for a first user.

diff --git a/tests/artifacts/PURPOSE b/tests/artifacts/PURPOSE
new file mode 100644
--- /dev/null
+++ b/tests/artifacts/PURPOSE
@@ -0,0 +1,9 @@
+This directory is meant to cache artifacts useful for tests (such as bundle).
+
+Those artifacts need to be cached because they are slow to regenerate on each
+test but too large to be tracked within the repository. They are not expected
+to change between each run and can be cached.
+
+The `./scripts/` contains code to generate the artifact while the `cache`
+directory contains resulting artifact.
+