is_unambiguous: fix false negatives again

Reported by Simon Jantsch and David Müller.

* spot/twaalgos/isunamb.cc (is_unambiguous): Rewrite wihtout assuming
that the product of two accepting SCCs is accepting,  Also use
the result of is_accepting_scc()/is_rejectng_scc() when available.
* spot/twaalgos/sccinfo.cc, spot/twaalgos/sccinfo.hh: Make it
possible to check the acceptance of a unique SCC.
* tests/core/unambig.test: Add more test cases.

tests/core/unambig.test

@@ -285,6 +285,7 @@ EOF
 run 1 autfilt -q --is-unambiguous smaller.hoa
 
 # These automata come from Simon Jantsch and David Müller.
+# They exposed bugs in the optimized version of our unambiguous check.
 cat >sjdb.hoa <<EOF
 HOA: v1
 name: "G(!a | !b) & (Ga | G(b | XGb))"
@@ -332,6 +333,51 @@ State: 3
 State: 4 {0}
 [!0&1] 4
 --END--
+HOA: v1
+States: 3
+Start: 0
+AP: 2 "p0" "p1"
+acc-name: Buchi
+Acceptance: 1 Inf(0)
+properties: trans-labels explicit-labels trans-acc
+--BODY--
+State: 0 "((G (! (p0))) R ((! (p1)) | (G (! (p0)))))"
+[!0] 1 {0}
+[0&!1] 0 {0}
+[!0&!1] 2
+State: 1 "(G (! (p0)))"
+[!0] 1 {0}
+State: 2 "((G (! (p0))) R ((! (p1)) | (G (! (p0))))) & (F (p0))"
+[0&!1] 0 {0}
+[!0&!1] 2
+--END--
 EOF
 
-test 2 = `autfilt -c --is-unambiguous sjdb.hoa`
+test 3 = `autfilt -c --is-unambiguous sjdb.hoa`
+
+# This automaton requires emptiness of SCC with Fin-acceptance.
+cat >ambig.hoa<<EOF
+HOA: v1
+States: 5
+Start: 4
+AP: 2 "p0" "p1"
+Acceptance: 4 Fin(0) & Fin(1)
+--BODY--
+State: 0
+[0] 0 {0}
+[!0] 1
+State: 1
+[1] 1 {1}
+[!1] 0
+State: 2
+[0] 2 {0}
+[!0] 3
+State: 3
+[1] 3 {1}
+[!1] 2
+State: 4
+[0] 0
+[0] 2
+--END--
+EOF
+test 0 = `autfilt -c --is-unambiguous ambig.hoa`