cmd/dist: simplify old code
Now that all uses of "go test" have been converted over to the new
abstraction, we can delete the old helpers for building "go test"
commands and simplify some code that's only used by the new
abstraction now.
For #37486.
Change-Id: I770cd457e018160d694abcc0b6ac80f7dc2e8425
Reviewed-on: https://go-review.googlesource.com/c/go/+/450020
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>