testcases: do not allow warnings in testsuccess

testcases: do not allow warnings in testsuccess

Adds a new testwarning which tests for zero exit and the presents of a
warning in the output, failing if either is not the case or if an error
is found, too. This allows us to change testsuccess to accept only
totally successful executions (= without warnings) which should help
finding regressions.

Git-Dch: Ignore