Make test work again and turn it on

Three assumptions were missing. Now the test works and we can turn
it back on.

Author: Owen Jones

regression/cbmc/unsigned___int128/main.c

@@ -29,6 +29,9 @@ void reduce(
     __CPROVER_assume(in[1]<((widelimb)1<<126));
     __CPROVER_assume(in[2]<((widelimb)1<<126));
     __CPROVER_assume(in[3]<((widelimb)1<<126));
+    __CPROVER_assume(in[4]<((widelimb)1<<126));
+    __CPROVER_assume(in[5]<((widelimb)1<<126));
+    __CPROVER_assume(in[6]<((widelimb)1<<126));
 
     static const widelimb two127p15 = (((widelimb) 1) << 127) +
         (((widelimb) 1) << 15);

regression/cbmc/unsigned___int128/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.c
 --unsigned-overflow-check --signed-overflow-check --function reduce
 ^EXIT=0$