Disabled by default since it requires `nix-daemon` binary
and Linux namespaces support.
For development this can be enabled by
```
cabal configure --flag=io-testsuite
```
or by adding
```
flags: +io-testsuite
```
to `cabal.project.local`
Enabled by `callCabal2nixWithOptions` in `overlay.nix` so
it is tested by `nix-build` and `nix-shell` brings all
test dependencies.
This is fine on NixOS where `build-tool-depends: nix:nix-daemon` works
and we have namespaces supported.