Problem with adding custom post type meta to header by plugin

I’m trying to write a plugin by which a meta code will be added to specific post type header.
I tried to use this code:

function msc_header() {
if ( is_singular( 'article' ) ) {
        echo '<meta name="site_title" content="'.'test test'.'">'."\n";
        }
}
add_action('wp_head','msc_header', 0);

But nothing happened. I got no error but no code was added. Am I missing something here?

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