xpath tests only pass in Arizona

Looks like a timezone issue.

Related:

* https://github.com/getodk/web-forms/issues/324

# Passing in Phoenix

```sh
$ TZ=America/Phoenix CI=true yarn workspace @getodk/xpath test
...
Test Files 74 passed | 2 skipped (76)
Tests 2530 passed | 25 skipped (2555)
Start at 09:36:39
Duration 106.37s (transform 2.00s, setup 70.05s, collect 2.64s, tests 22.31s, environment 152.81s, prepare 23.36s)

Done in 108.73s.
Done in 109.12s.
```

# Failing in LA

```sh
$ TZ=America/Los_Angeles CI=true yarn workspace @getodk/xpath test
...
Test Files 5 failed | 69 passed | 2 skipped (76)
Tests 36 failed | 2484 passed | 25 skipped (2545)
Start at 09:37:06
Duration 141.15s (transform 3.70s, setup 91.57s, collect 4.48s, tests 44.93s, environment 194.07s, prepare 30.99s)

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed.
Exit code: 1
```

# Failing in Greenwich

```sh
$ TZ=GMT CI=true yarn workspace @getodk/xpath test-node:jsdom
...
Test Files 7 failed | 67 passed | 2 skipped (76)
Tests 46 failed | 2467 passed | 25 skipped (2538)
Start at 16:37:23
Duration 132.48s (transform 5.01s, setup 90.72s, collect 4.92s, tests 43.99s, environment 175.99s, prepare 29.68s)

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed.
Exit code: 1
```

Start by running the @getodk/xpath test commands shown with America/Phoenix, America/Los_Angeles, and GMT, then compare the failures to identify the timezone-dependent tests. Trace the affected XPath test setup and make the suite produce consistent results across those timezones; done means the tests pass in each environment.

Written by the indexing model from the issue text.