The change in commit 3b95ab56 (Performance: Improve efficiency of
source file lookup in cmMakefile, 2017-08-17) broke some legacy behavior
of source file properties in which the order sources are first resolved
with extensions affects how setting properties without extensions works.
It has been reverted for now, but the discovery was made after merging
because the broken case was not covered by our test suite.

Add a test case representing the legacy behavior.

Issue: #15208