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.

TODO

@@ -37,10 +37,6 @@ Unless we play it dumb (little structure).
 
 Use "•" instead of ".".
 
-*** Conflict coverage
-Not all the conflicts have counterexamples generated.  See the "break"s in
-counterexample_report_state.
-
 *** point, dot
 We have too many names.  In XML we use "point", in C "dot".  A traditional
 more accurate name of "item" is "pointed rule", so we should probably prefer