Add test for Layout::write()

Add test for Layout::write()

When running the test, you'll get the following messages:

Testing ../../src/../lib/layouts/siamltex.layout...
Layout.cpp (268): Cannot copy unknown style `Enumerate'
Testing ../../src/../lib/layouts/svglobal.layout...
Cannot delete style `Dedication'
Testing ../../src/../lib/layouts/svjog.layout...
Cannot delete style `Dedication'
Testing ../../src/../lib/layouts/svprobth.layout...
Cannot delete style `Dedication'

These are no errors of Layout::write(), but they indicate problems in the
definition of the layout files.

Kornel, it would be nice if you could do the cmake part.