[CMake] Fix incorrect handling of get_target_property failure

Summary:
add_unittest was checking that the result of get_target_property was not
"NOTFOUND", but despite what the documentation says, get_target_property
returns <the var>-NOTFOUND on failure.

Reviewers: efriedma, thakis, serge-sans-paille, chandlerc

Reviewed By: serge-sans-paille

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81762