test: switch fieldtrack test to use GOEXPERIMENT

Now that we can set GOEXPERIMENT at build time, we no longer need
-d=fieldtrack in the compiler to enabled field tracking at build time.
Switch the one test that uses -d=fieldtrack to use GOEXPERIMENT
instead so we can eliminate this debug flag and centralize on
GOEXPERIMENT.

Updates #42681.

Change-Id: I14c352c9a97187b9c5ec8027ff672d685f22f543
Reviewed-on: https://go-review.googlesource.com/c/go/+/302969
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

test/fixedbugs/issue42686.go

@@ -1,4 +1,4 @@
-// compile -d=fieldtrack
+// compile -goexperiment fieldtrack
 
 // Copyright 2020 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style

test/run.go

@@ -513,7 +513,7 @@ func (t *test) run() {
 		return
 	}
 
-	var args, flags []string
+	var args, flags, runenv []string
 	var tim int
 	wantError := false
 	wantAuto := false
@@ -572,6 +572,9 @@ func (t *test) run() {
 			if err != nil {
 				t.err = fmt.Errorf("need number of seconds for -t timeout, got %s instead", args[0])
 			}
+		case "-goexperiment": // set GOEXPERIMENT environment
+			args = args[1:]
+			runenv = append(runenv, "GOEXPERIMENT="+args[0])
 
 		default:
 			flags = append(flags, args[0])
@@ -628,6 +631,7 @@ func (t *test) run() {
 		if tempDirIsGOPATH {
 			cmd.Env = append(cmd.Env, "GOPATH="+t.tempDir)
 		}
+		cmd.Env = append(cmd.Env, runenv...)
 
 		var err error