Feat test n option

[Mohamed-Moumni]

Summary

• Implement the -N file test operator (TsModif), which was previously unsupported and returned an error
• Add interp/actime package with platform-specific GetAtime() functions (darwin, linux, windows) to retrieve file access time, as access time retrieval differs across platforms
• Tested GetAtime() across all supported platforms, including Windows

Implemented #1316

[mvdan]

We can't just support the three main GOOSes; e.g. CI also covers js and plan9 because some users care about less widespread platforms. At least there should be a fallback file for other platforms that acts as a no-op by returning the mod time.

We need the type assertions to use the _, ok syntax and fall back to the mod time if the type doesn't match, because a custom handler may return an implementation of fs.FileInfo that doesn't contain this type in Sys.

Finally, name these files consistently with os_unix.go, i.e. os_linux.go, os_darwin.go, and so on.