cmd/go: write test file to temporary directory

Before this change, a go-vendor-issue-14613 file would be left in the
working directory after tests run.

Change-Id: If1858421bb287215ab4a19163f489131b2e8912c
Reviewed-on: https://go-review.googlesource.com/22169Reviewed-by: Chris Broadfoot <cbro@golang.org>

src/cmd/go/vendor_test.go

@@ -241,7 +241,7 @@ func TestVendorTest3(t *testing.T) {
 	tg.setenv("GOPATH", tg.path("."))
 	tg.run("get", "github.com/clsung/go-vendor-issue-14613")
 
-	tg.run("build", "-i", "github.com/clsung/go-vendor-issue-14613")
+	tg.run("build", "-o", tg.path("a.out"), "-i", "github.com/clsung/go-vendor-issue-14613")
 
 	// test folder should work
 	tg.run("test", "-i", "github.com/clsung/go-vendor-issue-14613")