cex: display all the S/R conflicts, not just one per (state, rule)

Before this commit, on

    %%
    exp
    : "if" exp "then" exp
    | "if" exp "then" exp "else" exp
    | exp "+" exp
    | "num"

we used to not display the third counterexample below:

    Shift/reduce conflict on token "+":
      Example              exp "+" exp . "+" exp
      First derivation     exp ::=[ exp ::=[ exp "+" exp . ] "+" exp ]
      Second derivation    exp ::=[ exp "+" exp ::=[ exp . "+" exp ] ]

    Shift/reduce conflict on token "else":
      Example              "if" exp "then" "if" exp "then" exp . "else" exp
      First derivation     exp ::=[ "if" exp "then" exp ::=[ "if" exp "then" exp . ] "else" exp ]
      Second derivation    exp ::=[ "if" exp "then" exp ::=[ "if" exp "then" exp . "else" exp ] ]

    Shift/reduce conflict on token "+":
      Example              "if" exp "then" exp . "+" exp
      First derivation     exp ::=[ exp ::=[ "if" exp "then" exp . ] "+" exp ]
      Second derivation    exp ::=[ "if" exp "then" exp ::=[ exp . "+" exp ] ]

    Shift/reduce conflict on token "+":
      Example              "if" exp "then" exp "else" exp . "+" exp
      First derivation     exp ::=[ exp ::=[ "if" exp "then" exp "else" exp . ] "+" exp ]
      Second derivation    exp ::=[ "if" exp "then" exp "else" exp ::=[ exp . "+" exp ] ]

* src/counterexample.c (counterexample_report_state): Don't stop of
the first conflicts.
* tests/conflicts.at, tests/counterexample.at, tests/diagnostics.at,
* tests/report.at: Adjust.

tests/conflicts.at

@@ -1758,6 +1758,14 @@ State 4
       Second example           resolved_conflict . 'a' 'a'
       Second derivation        start ::=[ resolved_conflict reported_conflicts ::=[ . 'a' ] 'a' ]
 
+    Shift/reduce conflict on token 'a':
+       10 reported_conflicts: . %empty
+        9 reported_conflicts: . 'a'
+      First example            resolved_conflict . 'a'
+      First derivation         start ::=[ resolved_conflict reported_conflicts ::=[ . ] 'a' ]
+      Second example           resolved_conflict . 'a' 'a'
+      Second derivation        start ::=[ resolved_conflict reported_conflicts ::=[ . 'a' ] 'a' ]
+
 
 
 State 5