gnulib-tool.py: Fix errors when executing --create-megatestdir. * pygnulib/GLTestDir.py (GLMegaTestDir.execute): Create a testdir for each module in its own subdirectory. Create a testdir with all modules in 'ALL'. Add missing join call on str object. Don't emit 'configure.ac' into 'Makefile.am'. Make sure commands are executed in the proper directory.