Add test demonstrating a position overlap bug.

Currently, whenever we call index_terms multiple times for a single
field, the term generator is being reset to position 0 each time. This
means that with text such as:

	To: a@b.c, x@y.z

one can get a bogus match by searching for:

	To: a@y.c

Thanks to Mark Anderson for reporting the bug, (and providing a nice,
minimal test case that inspired what is used here).