I guess it depends on why you are taking testosterone and how much of it you are taking. Women who are taking testosterone to have a more masculine appearance take high doses of testosterone; such a high level causes menstruation to cease. If you are talking about using testosterone gel to increase sexual desire, I don't know how much testosterone is being absorbed into the body, so I don't know if it is enough to cause changes in menstruation. It is possible, though.
Pregnancy tests work by detecting increased amounts of HCG. So testosterone would not affect a pregnancy test or cause a false result.