test bundle: Add demo-in-test-bundle game spec

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/examples/test-bundle/specs/demo-in-test-bundle.game.toml b/examples/test-bundle/specs/demo-in-test-bundle.game.toml
new file mode 120000 (symlink)
index 0000000..b61f2aa
--- /dev/null
+++ b/examples/test-bundle/specs/demo-in-test-bundle.game.toml
@@ -0,0 +1 @@
+../../../specs/demo.game.toml
\ No newline at end of file