alternation: fix detection of non-weak automata

Fixes #218.

* spot/twaalgos/alternation.cc: Adjust check.
* tests/core/alternating.test: Add test case from #218.
* NEWS: Mention the bug.

tests/core/alternating.test

@@ -374,5 +374,25 @@ State: 2 "t"
 [t] 2
 --END--
 EOF
-autfilt --tgba in >out 2>&1 && exit 1
+autfilt --tgba in 2>out && exit 1
+grep 'autfilt.*weak.*alternating' out
+
+cat >in <<EOF
+HOA: v1
+States: 4
+AP: 2 "a" "b"
+Start: 0&1
+Acceptance: 1 Inf(0)
+--BODY--
+State: 0 {0}
+[0] 2
+State: 2 /*{0}*/
+[0] 0
+State: 1 {0}
+[1] 3
+State: 3 /*{0}*/
+[1] 1
+--END--
+EOF
+autfilt --tgba in 2>out && exit 1
 grep 'autfilt.*weak.*alternating' out