Common code to setup and run sandbox is moved to testutil. Also, don't
link "boot" and "gofer" commands with test binary. Instead, use runsc
binary from the build. This not only make the test setup simpler, but
also resolves a dependency issue with sandbox_tests not depending on
container package.
PiperOrigin-RevId: 199164478
Change-Id: I27226286ca3f914d4d381358270dd7d70ee8372f