Filesystem API: Add more specificity to the rules for valid files in `validate_file()`.

This now treats files containing `./` as valid, and also treats files containing a trailing `../` as valid due to widespread use of this pattern in theme and plugin zip files.

Adds tests.

Props Ipstenu, borgesbruno, DavidAnderson, philipjohn, birgire
Fixes #42016, #36170

Built from https://develop.svn.wordpress.org/trunk@42011


git-svn-id: https://core.svn.wordpress.org/trunk@41845 1a063a9b-81f0-0310-95a4-ce76da25c4cd