She won't get popped for a failed drug test. If by chance she does, Serena could always say she was framed by the deep state, that the test was invalid because her sample was taken by an immigrant from a 3rd world sh*thole, she could claim that the liberal media is making up fakes news about her in an effort to set her up, She could distract attention from her failed test by criticizing Hillary's emails. She could claim the lab where the test was completed faked the test results in an effort to smear her good name. You know all of the excuses Trump and all of his loyal Trumptards on this forum would think were perfectly acceptable excuses.
If all else fails Serena could just say she likes Trump and millions of Trumptards will make all of these excuses for her.