Update shippable shared dir for COPY_SOURCE. (#16511)

Tests now use '/shared' instead of '/tmp/shared-dir' when using
COPY_SOURCE. This avoids issues with containers purging '/tmp'.

(cherry picked from commit fbfadc47c7)

test/utils/shippable/integration.sh

@@ -22,7 +22,7 @@ else
 fi
 
 if [ "${copy_source}" ]; then
-    test_shared_dir="/tmp/shared-dir"
+    test_shared_dir="/shared"
 else
     test_shared_dir="${test_ansible_dir}"
 fi