Realized in #125808 (comment).

A test writer might accidentally write

// @ignore-cross-compile

which is parsed as a normal comment not a compiletest directive because compiletest directives require that the line starts with //@.

This typo is hard to spot: both by PR authors and the reviewers. It might not fail in PR CI -- it might not even fail in full build CI, but then explode when the conditions are just right.

We should detect these and issue at least warnings from compiletest to help test writers realize the problem.