remfin: make sure Rabin automata are always converted to Büchi

Because using multiple acceptance condition is pointless in this case.

* src/twaalgos/remfin.cc (ra_to_ba): Extract most of the RA->BA code
into this new function for clarity.
* src/tests/remfin.test: Adjust.

src/tests/remfin.test

@@ -323,7 +323,8 @@ HOA: v1
 States: 15
 Start: 13
 AP: 2 "p1" "p0"
-Acceptance: 2 Inf(1) | Inf(0)
+acc-name: Buchi
+Acceptance: 1 Inf(0)
 properties: trans-labels explicit-labels state-acc
 --BODY--
 State: 0 {0}
@@ -398,7 +399,7 @@ State: 13
 [0&!1] 11
 [!0&1] 2
 [0&1] 3
-State: 14 {1}
+State: 14 {0}
 [!0&1] 14
 [0&1] 14
 --END--