test/codegen: mention in README that tests only run on Linux without -all_codegen

test/codegen: mention in README that tests only run on Linux without -all_codegen

This took me a while to figure out. The relevant code is in
test/run.go (note the "linux" hard-coded strings):

	var arch, subarch, os string
	switch {
	case archspec[2] != "": // 3 components: "linux/386/sse2"
		os, arch, subarch = archspec[0], archspec[1][1:], archspec[2][1:]
	case archspec[1] != "": // 2 components: "386/sse2"
		os, arch, subarch = "linux", archspec[0], archspec[1][1:]
	default: // 1 component: "386"
		os, arch, subarch = "linux", archspec[0], ""
		if arch == "wasm" {
			os = "js"
		}
	}

Change-Id: I92ba280025d2072e17532a5e43cf1d676789c167
Reviewed-on: https://go-review.googlesource.com/c/go/+/222819
Reviewed-by: Keith Randall <khr@golang.org>