The CMake configuration stopped creating any of the test builds. There
were a couple of errors introduced into this function that needed to be
fixed.

First, the condition on whether the testing should be skipped got
reversed. It was trying to create test build files for all the headers
that were not supposed to be built instead of the ones that were.

Second, the list of source files to build was not created correctly. The
set command that stored the name of the source file got removed, and so
nothing was being added to the list. I restored the variable and used
that consistently for the source file name.