The strep test is a vaginal swab. It does not dilate the cervix (and does not even generally enter it) and thus does not make a woman niddah. One should not perform a bedikah after this exam. There is no halachic obligation to do so.

A manual examination to check for dilation does not make a woman niddah either, unless the physician states that he is stripping the membranes (which he is not likely to do at 35 weeks).

B'sha'ah tovah!