fix(#324): runs xpath tests with the same timezone

[latin-panda]

Closes #324

I have verified this PR works in these browsers (latest versions):

• Chrome
• Firefox
• Safari (macOS)
• Safari (iOS)
• Not applicable

What else has been done to verify this works as intended?

To reproduce the error:

1. Change the machine time to March 1st at 23:30 Los Angeles timezone
2. Run the test suite yarn workspace @getodk/xpath test
3. See the tests failing like this:

Other things done:

After implementing the fix, I tried setting my machine in different timezones, ran the tests, and they passed. The different timezones have been passed to the "now" function correctly.

The offset is being appended to the localDateTimeString

I tried making a test about DTS, but it's hard to trick reliably the test to take a different DTS of the same timezone. However, I added some debugging code in localDateTimeString function, and I can see the offset switching to/from DTS for the same timezone.

// The process.env.TZ value is America/New_York
// Current date is March 3rd

console.log(dateTime.offset); // Prints "-05:00"
const dstDate = Temporal.ZonedDateTime.from("2025-07-01T23:30:00[America/New_York]");
console.log(dstDate.offset); // Prints "-04:00"

Why is this the best possible solution? Were any other approaches considered?

This ensures the tests are run in the same timezone by setting it in the beforeEach hook using the useFakeTimers utility. However, this doesn't mock the DST, which has been challenging to tick the system timer.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

N/A

Do we need any specific form for testing your changes? If so, please attach one.

I don't think so

What's changed

• Adds a global beforeEach and AfterEach hook to set the tests' timezone
• Adds a new optional environment variable for the tests' locale

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[latin-panda]

I didn't find the usage of this IMPLEMENTATION. I wanted to add a JSDoc description.

[latin-panda]

@eyelidlessness When you have a chance, can you please review it? Thank you!

[latin-panda]

This was added for completeness and to clarify that the tests expect the date format in this locale.
When switching to another locale (e.g., es-AR or es-MX), some tests fail because they expect a different date format.

With es-MX the now() returns a 1-1-1970 date, and tests using today() fail.
With es-AR the now() returns the current date correctly. But other tests fails, where tomorrow is set in en-US standard 2025-03-05

 FAIL  test/xforms/date-comparison.test.ts > date comparison > tomorrow > should be greater than or equal to today()
AssertionError: expected false to deeply equal true

- Expected
+ Received

- true
+ false