Fix typed_test to prevent fails in malloc_explicitly_typed (64-bit)

Fix typed_test to prevent fails in malloc_explicitly_typed (64-bit)

Issue #166 (bdwgc).

* tests/test.c (typed_test): Pass 320*sizeof(word)+some_number instead
of 2000 to GC_malloc_explicitly_typed, so that to satisfy the size
argument requirement of the latter regardless of word size).