Solidity: Prerelease warning made warning tests ineffective

[chriseth]

Due to the fact that every prerelease version of a compiler emits a warning for every input, the tests that check for the existing of some warning are now ineffective. We should change these tests to check the actual warning text.

[axic]

This should be done for the errors too - we want to check for specific errors and not "it's fine if any error has happened".

[chriseth]

@pirapira your PR only said "trying to fix". Do you want this to be reopened?

[pirapira]

Yes. The PR started to fix the problem in only one file. I knew the comment would close the issue, but I was too optimistic.

[pirapira]

This has been done by now.