tests: added test for timezone_select function in Date Helper

[ddevsr]

Description
Added tests for timezone_select() in Date Helper.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

💪

[kenjis]

This is outside the scope of this PR, but there are helpers with a newline code of "\n" and helpers with PHP_EOL, which is inconsistent.

It seems "\n" is better, because we use "\n" for view files.

[MGatner]

PHP_EOL is "\r\n" on Windows. Generally:

• Use \n for processing input
• Use PHP_EOL for CLI output
• Use PHP_EOL for writing files (unless the format is specified)
• Use either for web output

If we're looking for a consistent choice for web I'm with @kenjis on "\n".