cmd/compile: delete temporary files when test finishes

I noticed some files prefixed with ssa_fg_tmp in the /tmp folder of
the s390x builder. runGenTest (a helper for TestGenFlowGraph) wasn't
deleting its temporary files. The distinct prefix made this easy to
figure out.

Change-Id: If0d608aaad04a414e74e29f027ec9443c626e4eb
Reviewed-on: https://go-review.googlesource.com/78475Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/ssa_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"internal/testenv"
 	"io/ioutil"
+	"os"
 	"os/exec"
 	"path/filepath"
 	"runtime"
@@ -64,6 +65,7 @@ func runGenTest(t *testing.T, filename, tmpname string, ev ...string) {
 	if ok != nil {
 		t.Fatalf("Failed to create temporary directory")
 	}
+	defer os.RemoveAll(tmpdir)
 
 	rungo := filepath.Join(tmpdir, "run.go")
 	ok = ioutil.WriteFile(rungo, stdout.Bytes(), 0600)