tests/15 mixed pch: Add a test using only the headers
tests/15 mixed pch: Add a test using only the headers

Which is supposed to work for GCC and Clang.

becm/meson,pexip/meson,thiblahute/meson,jeandet/meson,jeandet/meson,QuLogic/meson,MathieuDuponchelle/meson,jpakkane/meson,MathieuDuponchelle/meson,becm/meson,jeandet/meson,MathieuDuponchelle/meson,pexip/meson,jeandet/meson,pexip/meson,jeandet/meson,mesonbuild/meson,MathieuDuponchelle/meson,mesonbuild/meson,mesonbuild/meson,thiblahute/meson,MathieuDuponchelle/meson,pexip/meson,pexip/meson,jpakkane/meson,jpakkane/meson,MathieuDuponchelle/meson,thiblahute/meson,mesonbuild/meson,MathieuDuponchelle/meson,thiblahute/meson,MathieuDuponchelle/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,becm/meson,pexip/meson,pexip/meson,jpakkane/meson,MathieuDuponchelle/meson,jpakkane/meson,becm/meson,becm/meson,thiblahute/meson,becm/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,becm/meson,mesonbuild/meson,becm/meson,jeandet/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,thiblahute/meson,QuLogic/meson,mesonbuild/meson,thiblahute/meson,jeandet/meson,QuLogic/meson,pexip/meson,pexip/meson,becm/meson,thiblahute/meson,jeandet/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,QuLogic/meson,becm/meson,pexip/meson,thiblahute/meson,jeandet/meson