cmd/go: add check for unknown godebug setting

A //go:debug line mentioning an unknown or retired setting
should be diagnosed as making the program invalid. Do that.
We agreed on this in the proposal but I forgot to implement it.

Change-Id: Ie69072a1682d4eeb6866c02adbbb426f608567c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/476280
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/internal/godebug/godebug_test.go

@@ -11,13 +11,13 @@ import (
 )
 
 func TestGet(t *testing.T) {
-	foo := New("foo")
+	foo := New("#foo")
 	tests := []struct {
 		godebug string
 		setting *Setting
 		want    string
 	}{
-		{"", New(""), ""},
+		{"", New("#"), ""},
 		{"", foo, ""},
 		{"foo=bar", foo, "bar"},
 		{"foo=bar,after=x", foo, "bar"},
@@ -28,7 +28,7 @@ func TestGet(t *testing.T) {
 		{"foo=", foo, ""},
 		{"foo", foo, ""},
 		{",foo", foo, ""},
-		{"foo=bar,baz", New("loooooooong"), ""},
+		{"foo=bar,baz", New("#loooooooong"), ""},
 	}
 	for _, tt := range tests {
 		t.Setenv("GODEBUG", tt.godebug)