Remove a footgun-y feature / relic of the past from the compiletest DSL
The compiletest DSL still features a historical remnant from the time when its directives were merely prefixed with `//` instead of `//`@`` when unknown directive names weren't rejected since they could just as well be part of prose:
As an "optimization", it stops looking for directives once it stumbles upon a line which starts with either `fn` or `mod`. This is super footgun-y as it obviously leads to any seeming compiletest directives below `fn` and `mod` items getting completely ignored.
See #136403 for a practical example. As well the assembly test updated in this PR.
~~Blocked on #136403.~~ (merged)
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