Move stdlib to its own place

The stdlib tests aren't really scenario tests, and I also want to
expand them to run across multiple versions of Go (ignoring releases
before 1.0.3 as withmock/mocktest don't work with those versions).
Fix typo
Add more context errors
Don't try and mock the testing package
stdlib test script for testing single go version
Only set the env for commands we run

Don't change our own environment, only the environment of commands
that we run.
Add support for compilation mode

Add a flag that will allow the user to run mocktest in compilation
mode (i.e. call go test -c to just build the binary).  This means that
we can't fix the output for them (or rather we don't, as I can't be
bothered to go to the trouble of modifying the path strings in the
binary at the moment).

Also, we allow go test to tell the user that multiple packages aren't
supported currently - it might be nicer to do that check ourselves, as
then we won't waste time setting everything up.