cmd/go: add test checking version with experiment is valid

For #75953

Change-Id: I6a6a69645d6d2af9be1b076f2460cdb295ea3c6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/720600
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/testdata/script/version_goexperiment.txt

@@ -12,5 +12,13 @@ stderr 'X:fieldtrack$'
 
 -- version.go --
 package main
-import "runtime"
+import (
-func main() { println(runtime.Version()) }
+	"go/version"
+	"runtime"
+)
+func main() {
+	if !version.IsValid(runtime.Version()) {
+		panic("version not valid: "+runtime.Version())
+	}
+	println(runtime.Version())
+}