No one ever runs 'go tool api', because the invocation
has gotten unwieldy enough that it's not practical.
And we don't support it as a standalone tool for other
packages - it's not even in the distribution.
Making it an ordinary package test lets us invoke it
more easily from cmd/dist (as go test cmd/api -check)
and avoids the increasingly baroque code in run.go to
build a command line.
Left in cmd/api even though it's no longer a command
because (1) it uses a package from cmd/vendor and
(2) it uses internal/testenv. Otherwise it could be misc/api.