[11.x] improvement tests for Str::kebab method

[saMahmoudzadeh]

This PR enhances the test coverage for the Str::kebab method.

using space in $value input:

$this->assertSame('laravel-php-framework', Str::kebab('Laravel Php Framework'));

using special character(❤) in $value input:

$this->assertSame('laravel❤-php-framework', Str::kebab('Laravel ❤ Php Framework'));

if $value is empty string

$this->assertSame('', Str::kebab(''));