Add tests for two bugs that were incidentally fixed some time ago.
Commit b894cb41d869bda6ba0c54a491becc7bb58375c1 (make FORMAT signal
errors with illegal COLINC etc. parameters) happened to fix bug 905817
(endless loop in FORMAT with zero COLINC), then unnoticed, so add a
regression test and NEWS now.
Commit 96c62c30ec9164419c790b2fbea953da2193620f (ensure that GCD returns
positive values) besides the one intended also fixed bug 516750 (type
error when trying to convert a sum of rationals to a float). Add the
failing expression from that bug to the existing test.