fix IdnaTest.inc generation w/out of tree builds

The current rule will always look for IdnaTest.txt in the current directory (and output IdnaTest.inc to the current directory). When doing an out-of-tree build, the Makefile (via vpaths) will find the source files, but when the rule runs, it won't. Change the rules to use automatic makefile variables so it can deal with srcdir/builddir dynamically.

It does mean that always write IdnaTest.txt to the builddir and not the sourcedir, but I think that's what the code intends. When doing a make dist, it'll source the right file and save it to the right place in the tarball.