Exclude test for MMTk

Author: peterzhu2118

test/.excludes-mmtk/TestObjSpace.rb

@@ -1,4 +1,5 @@
 exclude(:test_dump_all_full, "testing behaviour specific to default GC")
 exclude(:test_dump_flag_age, "testing behaviour specific to default GC")
 exclude(:test_dump_flags, "testing behaviour specific to default GC")
+exclude(:test_dump_includes_slot_size, "can be removed when BASE_SLOT_SIZE is 32 bytes")
 exclude(:test_dump_objects_dumps_page_slot_sizes, "testing behaviour specific to default GC")