IF you use more than 1000mg testosterone a week from the get go, that is, when you are fertile, then you should in theory keep your fertility while on testosterone.
This is because sperm need high amounts of test to mature and develop. Normally the testes have a 100x higher test concentration than the rest of the body. When you start a cycle, even though your body/muscles will receive much more test than when natural, the testicles will actually be starved of testosterone due to the negative feedback loop.
It has been shown in rodents that sperm can mature without FSH and LH as long as test concentration remains high.
Thus test alone can keep fertility going.