alternation: better detection of non-weak alternating automata

* spot/twaalgos/alternation.cc: Fix the code to also check the
weakness of single-state SCCs.
* tests/core/alternating.test: Add a test from ltl3hoa.

tests/core/alternating.test

@@ -352,3 +352,27 @@ State: 0
 --END--
 EOF
 diff out5 expect
+
+# Detect cases where alternation-removal cannot work.
+cat >in <<EOF
+HOA: v1
+name: "SLAA for p1 & FGp1"
+States: 3
+Start: 0&1
+AP: 1 "p1"
+acc-name: co-Buchi
+Acceptance: 1 Fin(0)
+properties: univ-branch trans-labels explicit-labels trans-acc
+properties: deterministic
+--BODY--
+State: 0 "p1"
+[0] 2
+State: 1 "FGp1"
+[0] 1
+[!0] 1 {0}
+State: 2 "t"
+[t] 2
+--END--
+EOF
+autfilt --tgba in >out 2>&1 && exit 1
+grep 'autfilt.*weak.*alternating' out