tests: introduce c-style conditional sections in .t tests This makes it possible to have conditional sections like: #if windows $ echo foo foo #else $ echo bar bar #endif The directives and skipped sections are treated like comments, so don't interleave them with commands and their output. The parameters to #if are evaluated while preparing the test by passing them over to hghave. Requirements can thus be negated with 'no-' prefix, and multiple requirements must all be true to return true.