Don't hardcode the source filename

In order to compile these tests with different CXXFLAGS, I needed to copy them
to different filenames to trick the compiler. Unfortunately, this triggers a
failure with one of the tests as it hardcodes the path to the test, instead of
relying on the compiler to fill in the path via __FILE__.

Using __FILE__ is standard and works. Rely on it instead of a hardcoded path.