Improve Intl.DurationFormat tests

[anba]

d26a157

• Adds a simplified implementation of PartitionDurationFormatPattern to generate the expected results
• The implementation is partially based on the updates from Editorial: Various editorial updates proposal-intl-duration-format#166 and the SpiderMonkey patches

275c0ca

• Intl.DurationFormat currently requires exact mathematical computations. Test for this until there's an agreement to change this.
• Also see Decide if upcoming Temporal.Duration limits should have an impact on DurationFormat proposal-intl-duration-format#157.

2e7a522

• In Editorial: Various editorial updates proposal-intl-duration-format#166 I've mentioned a strange formatting case. This adds a test case for it.

89ecd6d

• Add missing coverage for negative durations.
• Also see: Format of negative durations proposal-intl-duration-format#169

[ben-allen]

Can I just say that this is truly incredible? Thank you so much for all your work here!

[FrankYFTang]

LGTM thanks for working on this.