fdo#72041: Add import test to catch an issue with external ref cache.

Change-Id: I648d2d04c86d61b1cbe291e7aa79bf9219833328
fdo#72041: Intern strings as we populate the external cache.

This commit covers ods import and UNO API layer.

Change-Id: I3ad1b7cfefe49575694e2351bcba6e5733b009d2