allow keeping $WORK after tests complete

[kortschak]

After the talk at GopherConAU, I took a look at rsc.io/script for comparison with the current testscript I'm using (Rog's testscript extracted from internal). One of the things that I've noticed is that as far as I can see, there is no way for the invoker of go test to ask for the work dir to be left after the tests complete. This is possible in the go-internal package and was possible in the original script test implementation in go/cmd. But is missing here because the temp dir used for work dir is created by testing.common.TempDir which is always deleted on exit.

Having a way to examine scratch work in failing tests is a very valuable tool to have.