Redirect sanity checks' output to temporary files (and archive them as artifact files under CircleCLI).

This should make it easier to find and access relevant debugging information without polluting STDOUT and without any risk of log interleaving. See also:
- https://weaveworks.slack.com/archives/net/p1482154505001192
- https://weaveworks.slack.com/archives/net/p1482254780001493

bin/circle-test-smoke

@@ -9,5 +9,7 @@ if [ -n "$TEST_AND_PUBLISH" ] ; then
     cd $SRCDIR/test
     eval $(./gce.sh hosts)
     export COVERAGE=true
+    export WEAVE_NET_SANITY_CHECKS_FILES="/tmp/weave_net_sanity_check_*.log"
     ./run_all.sh
+    cp $WEAVE_NET_SANITY_CHECKS_FILES $CIRCLE_ARTIFACTS
 fi