Test: fix ZIP entry LastWriteTime tests to support running locally (when system clock is not UTC)

[dtivel]

Bug

Fixes: NuGet/Home#11312

Description

This change fixes some tests that only pass when the system clock is UTC (like on a CI build machine).

The way the DateTimeOffset ZipArchiveEntry.LastWriteTime property works is for any DateTimeOffset value you set it to, ZipArchiveEntry will use only the DateTime property of that DateTimeOffset. DateTimeOffset.DateTime effectively returns the DateTime component of a DateTimeOffset, after first discarding its time zone information. To illustrate, the DateTime property of these different DateTimeOffset values all return the same DateTime value of 8/3/2024 11:50:51 AM with an "unspecified" time zone.

8/3/2024 11:50:51 AM -07:00
8/3/2024 11:50:51 AM +00:00
8/3/2024 11:50:51 AM +05:30

That means any test that checks that NuGet sets a file's LastWriteTime value correctly must use DateTimeOffset.DateTime to check the file's last write time in UTC, or DateTimeOffset.DateTime.ToLocalTime() to check the file's last write time in local.

As part of this change, I changed the entryModifiedTime type from DateTime to DateTimeOffset to match the ZipArchiveEntry.LastWriteTime data type.

I also opportunistically fixed some code style issues in the area.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests N/A
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. N/A

CC @aortiz-msft

[jeffkl]

Thanks for fixing this, I saw fail a few months ago and never got around to doing it.