Fix time parsing in gigasecond tests

[kahgoh]

As mentioned in #232, this is to fix the Gigasecond tests (failed in test run https://github.com/exercism/vlang/actions/runs/11506925109).

This fixes the parsing of the date strings in the tests. According to the docs the, a couple of the dates are in ISO 8601 format rather than RFC 3339. RFC 3339 requires the time zone to be specified when the String contains a time. ISO 8601 doesn't have this requirment.

See:

• https://modules.vlang.io/time.html#parse_rfc3339
• https://ijmacd.github.io/rfc3339-iso8601/

When merging, I think it should be marked [no important files changed] as this commit shouldn't change any past test results.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

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[kahgoh]

Btw, the break in the Gigasecond test seems to have been caused by this commit in Vlang: vlang/v@c55a75f