Don't use the if..  store the result and do an assertFalse/assertTrue on that result. 

In a test you normally have 2 phases:
1) setup sut (system under test) so that it is prepared for number 2.

2) doing the operation on the sut (preferably 1 method)

3) doing the assertions.

If you do control flow logic for 2/3 then there is no clear separation of them.