Add more tests for ceylon/ceylon-spec#869

Suggested by @chochos [1]. I added another test to verify that
conditions after a false condition are not evaluated:
    if (false) if (functionWithSideEffects()) value
should not have side effects.

[1]
https://github.com/lucaswerkmeister/ceylon.language/commit/f8ef15cc944987f78654ce1121187f820fcee08d#commitcomment-5126747