feat(wpt): enable tests for FormData

[huancheng-trili]

Context

Part of JSTZ-303.
JSTZ-303

Description

Enable test suites for FormData (interface, class).

Manually testing the PR

cargo test --test wpt

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.67%. Comparing base (3234d6d) to head (afe0e14).

see 1 file with indirect coverage changes

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