Improve the set up of the test objects by either adding an assert that a
valid uno::Reference<sheet::XSpreadsheetDocument> exists or adding a
message to existing asserts.

Change-Id: I21af3a168eeae232edc8ef6787c439279e52d266
Reviewed-on: https://gerrit.libreoffice.org/46176Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jens Carl <j.carl43@gmx.de>