This makes test launch their own isolated instance of containerd with
its own root/state directories and listening socket address. Each test
brings this instance up/down on its own.
Add options to override containerd address in the code (used only in the
tests).
Enable parallel go test runs once again.
P.S. I wish I could share that 'SetupSuite' phase across the tests, but
afaik there's no way in Go to share `_test.go` code across packages. If
we put it as normal package, this might pull in test dependencies (like
`testify`) into production code, which I don't like.
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
