test: use go tool from tree, not path

Some of our tests do exec.Command("go", "tool", "compile", ...) or
similar. That "go" is selected from PATH. When run.go is started
from the command line (but not from all.bash), the first "go" is whatever
happens to be first in the user's path (some random older version than
tip). We really want all these tests to use the "go" tool from the
source tree under test. Add GOROOT/bin to the front of the path to
ensure that the tools we use come from the source tree under test.

Change-Id: I609261a4add8cd5cb228316752d52b5499aec963
Reviewed-on: https://go-review.googlesource.com/c/go/+/418474
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

test/run.go

@@ -58,7 +58,7 @@ type envVars struct {
 }
 
 var env = func() (res envVars) {
-	cmd := exec.Command("go", "env", "-json")
+	cmd := exec.Command(goTool(), "env", "-json")
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
 		log.Fatal("StdoutPipe:", err)
@@ -710,6 +710,22 @@ func (t *test) run() {
 		if tempDirIsGOPATH {
 			cmd.Env = append(cmd.Env, "GOPATH="+t.tempDir)
 		}
+		// Put the bin directory of the GOROOT that built this program
+		// first in the path. This ensures that tests that use the "go"
+		// tool use the same one that built this program. This ensures
+		// that if you do "../bin/go run run.go" in this directory, all
+		// the tests that start subprocesses that "go tool compile" or
+		// whatever, use ../bin/go as their go tool, not whatever happens
+		// to be first in the user's path.
+		path := os.Getenv("PATH")
+		newdir := filepath.Join(runtime.GOROOT(), "bin")
+		if path != "" {
+			path = newdir + string(filepath.ListSeparator) + path
+		} else {
+			path = newdir
+		}
+		cmd.Env = append(cmd.Env, "PATH="+path)
+
 		cmd.Env = append(cmd.Env, runenv...)
 
 		var err error