glsl/glcpp: Integrate recent glcpp-test-cr-lf test into "make check"

glsl/glcpp: Integrate recent glcpp-test-cr-lf test into "make check"

Beyond just listing this in the TESTS variable in Makefile.am, only minor
changes were needed to make this work. The primary issue is that the build
system runs the test script from a different directory than the script
itself. So we have to use the $srcdir variable to find the test input files.

Using $srcdir in this way also ensures that this test works when using an
out-of-tree build.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>