Convert sandbox_bpf_unittest.cc to use bpf_dsl

Convert sandbox_bpf_unittest.cc to use bpf_dsl

This unfortunately means converting what are currently sandbox_bpf unit
tests into effectively integration tests for bpf_dsl, as they now test
both bpf_dsl + sandbox_bpf.  However, this is a necessary step towards
eliminating dependencies on the current sandbox_bpf API so that we can
more freely evolve it to fit bpf_dsl's needs.

For ease of review, this only converts tests that were already using
SandboxBPFPolicy (i.e., BPF_TEST()s are left alone), and some of the
conversions are slightly non-idiomatic in the interest of mimicking the
existing test structure.  Both of these will be addressed further in
followup CLs so that this CL can be reviewed as a mere translation of
policies from the sandbox_bpf API to bpf_dsl.

Review URL: https://codereview.chromium.org/559653004

Cr-Commit-Position: refs/heads/master@{#294723}