Run sandbox tests on Xenial and Bionic

We currently run a Tor Travis CI chutney job on Bionic with the sandbox disabled.

The chutney Travis CI runs on Bionic and Xenial (for pypy), also with the Sandbox disabled.

Once legacy/trac#32722 (moved) is fixed, we should run some sandbox unit tests, Tor binary tests, or chutney on both Xenial and Bionic, and newer Ubuntu versions as Travis creates images for them.

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