change to regrtest.py to allow optional running of test_largefile ('cause it's
slow on Win64).

This closes patches:
http://sourceforge.net/patch/index.php?func=detailpatch&patch_id=100510&group_id=5470
and
http://sourceforge.net/patch/index.php?func=detailpatch&patch_id=100511&group_id=5470