Small refactor in the validate_file method + unit tests

[borgesbruno]

The validate_file( $file, $allowed_files = '' ) method has as third param an array of allowed files, but the default value is a string, I created this patch using as default value an empty array instead of a empty string.

Beyond that, I wrote some unit tests for this change and to improve de code coverage.

[borgesbruno]

Patch 36170

[borgesbruno]

Using type hint and changing the provider

[borgesbruno]

Removing type hint

[borgesbruno]

I've removed the type hint, because is also necessary to do the refactor for
validate_file_to_edit method.

[borgesbruno]

Please review

[ocean90]

@borgesbruno Thanks for your patch and the unit tests.

Can you please adjust your patch according our ​coding standards?

Let's change the default value for validate_file_to_edit() too.

[borgesbruno]

Tks for your feedback @ocean90, to avoid theses small errors with codings standards, now I'm using the phpcs with WordPress standard mode.
I've fixed theses errors on the last patch, please, if you can review again =)

[johnbillion]

In 42007:

Docs: Improve the docs for validate_file() and validate_file_to_edit().

See #42016, #36170, #41017

[johnbillion]

In 42011:

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