tests: drop unused pipe_in calls and keep date checks valid across UTC midnight

[ChihweiLHBird]

Summary

test_relative_weekdays, test_date_numeric_d_basic_utc and test_date_military_timezone_with_offset_and_date read today's UTC date once and then run date several times against it. If UTC midnight passes in between, date is already on the next day and the test fails although nothing is wrong.

This PR reads the date before and after each date run. If the day changed, test_relative_weekdays runs that case again, since its input is computed from today, and the other two accept the result for either day. Otherwise the checks are as strict as before.

The second commit follows up on #14431: the four numfmt and sort tests it changed fail while parsing arguments and never read stdin, so pipe_in("") does nothing there, and without it the harness gives them /dev/null.

[Copilot]

🟢 Approval recommended

The changes are limited to test cleanup and midnight handling, with no blocking issues identified.

0 open findings

What changed in this PR

This test-only PR reduces UTC-midnight flakiness and removes unnecessary stdin setup.

Changes:

• Handle UTC date transitions in weekday, numeric-date, and military-timezone tests.
• Remove empty pipe_in calls from argument-validation tests.

File	Description
tests/​by-util/​test_sort.rs	Remove unnecessary empty stdin setup.
tests/​by-util/​test_numfmt.rs	Remove unnecessary empty stdin setup.
tests/​by-util/​test_date.rs	Make date assertions tolerate UTC-midnight transitions.

🧠 Review effort: Balanced

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