Fix BaseConfig didn't load Registrar files properly

[vibbow]

Description
In origional BaseConfig.php file, in registrProperties function, it use locator to find all Config/Registrar.php files in known namespace, and it returned all file path as an array.

In following code, it use the file path as a callable object, to check if registrars exists (by using method_exists) for current config file, which didn't work in my case.

I'm not sure if method_exists works with file path, but I updated this function to load all registrars files first before future usage.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[vibbow]

Wait a second, the tests is failed and it's related to this change.

In theory, it should discovery Registrar file at test/_support/Config/Registrar.php, and calling RegistrarConfig function, and the function did return an array.

Need to figure out why following code get triggered

if (! is_array($properties))
{
	throw new \RuntimeException('Registrars must return an array of properties and their values.');
}

[vibbow]

Get failed test fixed.