gotooltest: better cgo support

[mvdan]

I propose two changes to gotooltest to better support Go tests which use cgo:

1. A conditional, [cgo], which tells you whether cgo is enabled. Practically equivalent to the shell [[ $(go env CGO_ENABLED) == "1" ]]. To be used like [!cgo] skip 'this test requires cgo to work'.

2. Passing the CGO_ENABLED env variable through to test scripts, like GOOS or GOARCH. I can use GOARCH=386 go test to run my Go test scripts with that architecture, but note that cross-compilation in Go disables cgo by default. GOARCH=386 CGO_ENABLED=1 go test doesn't make a difference, as the latter env var isn't forwarded.

I can work around item 1 by implementing the conditional myself, and I have, but might as well have it built-in :)

I can work around item 2 by manually forwarding the env var via the Setup func, but that feels hacky. I can't simply env CGO_ENABLED=1 in the test, because that will go on to assume that cgo can work. Which is not always the case; for instance, GOARCH=386 go test fails on CI because the regular gcc is installed rather than gcc-multilib, and the former can't cross-compile from amd64 to 386 as it fails to find some header files.

Happy to submit the changes, I just thought I'd propose them first. cc @rogpeppe @myitcv