testnode: include a link to snapshot dir in log viewer

From there we can now access zLOG.log and Z2.log for erp5 and snapshots
from slapos software release tests

erp5/util/testnode/js-logtail/logtail.html

@@ -14,6 +14,7 @@
             <a href="logtail.html?noreverse">chronological</a> view.
             <a id="pause" href='#'>Pause</a>.
             <a href="suite.log">Full log file</a> or
+            <a href="./">snapshots directory</a>
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