fix: handle SIGHUP from OpenSSH (#10638)

Fixes an issue where remote forwards are not correctly torn down when using OpenSSH with `coder ssh --stdio`.  OpenSSH sends a disconnect signal, but then also sends SIGHUP to `coder`.  Previously, we just exited when we got SIGHUP, and this raced against properly disconnecting.

Fixes https://github.com/coder/customers/issues/327

testutil/ctx.go

@@ -11,3 +11,14 @@ func Context(t *testing.T, dur time.Duration) context.Context {
 	t.Cleanup(cancel)
 	return ctx
 }
+
+func RequireRecvCtx[A any](ctx context.Context, t testing.TB, c <-chan A) (a A) {
+	t.Helper()
+	select {
+	case <-ctx.Done():
+		t.Fatal("timeout")
+		return a
+	case a = <-c:
+		return a
+	}
+}