libc: Test time zone change detection.

[des]

Summary

Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.

[des]

nits

[des]

tzset is unnecessary and may slow the test down

[des]

more cleanup

[markj]

contrib/tzcode/localtime.c

412

Is this bringing us closer to upstream's code?

lib/libc/tests/stdtime/detect_tz_changes_test.c

56

Probably not much of an issue in practice, but I believe copy_file_range() must be called in a loop to handle short I/O.

153

I believe ATF_* can't be used in a child process.

[des]

contrib/tzcode/localtime.c

412

No, this code does not exist upstream.

lib/libc/tests/stdtime/detect_tz_changes_test.c

153

It works just fine. It emits an error message to stderr (which we echo) and kills the child (which causes the parent to fail).

[des]

contrib/tzcode/localtime.c

512–513

This actually fixes a regression in bc42155199b5. I'll split it out into a separate commit.

[des]

rebase