If you eat something with iron in it right now, your body won't have time to use it to make new red blood cells (if my science is off, anyone please correct me) so it won't affect your iron count on the test. However, if you do turn out to be anemic, you have options other than giving up on vegetarianism (which doesn' t cause anemia anyway): You can commit to eating iron-rich foods consistently (not just before tests) or you can take iron supplements. For me, it doesn't seem to matter how much iron I get in my diet, I still need the pills to keep in the normal range. (Of course, you shouldn't take the pills unless your doctor tells you to. Iron isn't water soluble so if you take in too much it can build up and cause other problems.)
Good luck on your test.