cmd/go: fix bug in test of go get ./path needing to download path

rsc.io/toolstash is gone; use rsc.io/pprof_mac_fix.

This fixes a bug in the test. It turns out the code being tested here
is also broken, so the test still doesn't pass after this CL (filed #19769).

Change-Id: Ieb725c321d7fab600708e133ae28f531e55521ad
Reviewed-on: https://go-review.googlesource.com/38743Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/go/go_test.go

@@ -2655,8 +2655,8 @@ func TestGoVetWithTags(t *testing.T) {
 	tg.grepBoth(`c\.go.*wrong number of args for format`, "go get vetpkg did not run scan tagged file")
 }
 
-// Issue 9767.
-func TestGoGetRscIoToolstash(t *testing.T) {
+// Issue 9767, 19769.
+func TestGoGetDotSlashDownload(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)
 
 	tg := testgo(t)
@@ -2664,7 +2664,7 @@ func TestGoGetRscIoToolstash(t *testing.T) {
 	tg.tempDir("src/rsc.io")
 	tg.setenv("GOPATH", tg.path("."))
 	tg.cd(tg.path("src/rsc.io"))
-	tg.run("get", "./toolstash")
+	tg.run("get", "./pprof_mac_fix")
 }
 
 // Issue 13037: Was not parsing <meta> tags in 404 served over HTTPS