Test that downloadCurrentVersion.ts actually downloads the latest version

[cameel]

We used to test that solcjs --version returns a version matching the one from package.json. I changed that in #626 because this is not really guaranteed - the returned version depends on the binary. So now we should also add a proper test that downloadCurrentVersion.ts actually does its job and downloads the right binary. It could be a JS test or an additional check in one of the jobs in CI.