Add shutdown call after each test to initialize a new sandbox env for every test.

Before this change, only the first APCT test that ran had a non-zero
SANDBOX_INIT_TIME_AVG value. With the change, all tests populate this
value since they create a new sandbox env.

Test: Running APCT tests. Results: https://paste.googleplex.com/4864291380396032
Change-Id: Ic6285a0695e7559181bf09ab4aca5e905f14a984