Update test expectations from application/javascript -> text/javascript

`package:mime` now uses `text/javascript` and not `application/javascript`.

See https://github.com/dart-lang/mime/pull/76.
See https://datatracker.ietf.org/doc/html/rfc9239.

> This document defines equivalent processing requirements for the various script media types. The most widely supported media type in use is `text/javascript`; all others are considered historical and obsolete aliases of `text/javascript`.