Replace OSL_VERIFY with if-statement and SAL_WARN and add some unit
tests to ensure the implementation works and the replacements don't
introduce some side effects.

Change-Id: I4ed50eee53e4dba4d392ed2486186d48f919ce1d
Reviewed-on: https://gerrit.libreoffice.org/71712
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>