TAW: Check that generated access widener matches expected output

[Juuxel]

See https://github.com/FabricMC/fabric/pull/2695#discussion_r1032199744.

This prevents builds with outdated access wideners when the MC version changes, or when template.accesswidener is modified.

Also simplifies the code a bit - text.lines().collect(Collectors.toCollection { [] }) for getting a mutable list of the lines in a file was replaced with Groovy's readLines extension method wrapped in a new ArrayList for safety.