internal/build:switch workdir from testpkg like go behavior

cl/_testgo/runtest/main_test.go

@@ -1,6 +1,9 @@
 package main
 
-import "testing"
+import (
+	"os"
+	"testing"
+)
 
 func TestZoo(t *testing.T) {
 	if Zoo() != 3 {
@@ -11,3 +14,10 @@ func TestZoo(t *testing.T) {
 func TestFalse(t *testing.T) {
 	// t.Fatal("false")
 }
+
+func TestSwitchWd(t *testing.T) {
+	_, err := os.Stat("./foo")
+	if err != nil {
+		t.Fatal(err)
+	}
+}

internal/build/build.go

@@ -508,6 +508,7 @@ func linkMainPkg(ctx *context, pkg *packages.Package, pkgs []*aPackage, conf *Co
 	switch mode {
 	case ModeTest:
 		cmd := exec.Command(app, conf.RunArgs...)
+		cmd.Dir = pkg.Dir
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr
 		cmd.Run()