This is the best we can do since the rest of the cell merge code is
in ScViewFunc which cannot be unit-tested currently.

Change-Id: I639f48d5cd05bff015689b0d223684f6f8d302f8
Reviewed-on: https://gerrit.libreoffice.org/37179Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>