Port 'cmd/go: add mv and support "! cmp" in script tests'

[myitcv]

From https://go-review.googlesource.com/c/go/+/380916

[mvdan]

I'll take a stab at this as I want ! cmp for https://github.com/burrowers/garble.

[bitfield]

Just one comment, and I realise this applies to upstream too: it's a little confusing that you can't mv files into a directory the same way you can with cp:

mkdir new

# works
cp a.txt b.txt new

# doesn't work
mv a.txt b.txt new

-- a.txt --
-- b.txt -- 

[myitcv]

Hmm I'd say that probably warrants a new issue to capture that. Feels like an oversight in the upstream implementation?