cmd/fix: move cgo and 'go build' support checks into TestRewrite

cmd/fix: move cgo and 'go build' support checks into TestRewrite

This avoids unnecessary work to determine 'go build' and cgo support
if we're not actually running the test (as in 'go test -list').

Change-Id: Id175a759605b2130d4de8bff8eba4c23fe65ccba
Reviewed-on: https://go-review.googlesource.com/c/go/+/491657
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Austin Clements <austin@google.com>