Use CXX_SOURCES and point to the right source file.

Copy paste error, but the test still built on macOS.  Weird.
It failed on debian linux with an error about -fno-limit-debug-info
not being a supported flag???  Not sure how this goof would cause
that error, but let's see if it did...

GitOrigin-RevId: 98feb08e449f179c3c5ccc6878c31cf16c160b06
1 file changed