Filtering Posts by Taxonomies in the Dashboard

admin_filter_dropdown_cpt_by_taxonomy.php

<?php

function filter_customcpt_by_taxonomies( $post_type, $which ) {

	// Replace it with your custom post type
	if ( 'custom_post_type' !== $post_type )
		return;

	// Here you can add how many taxonomy you want
	$taxonomies = array( 'testimonial_type' );

	foreach ( $taxonomies as $taxonomy_slug ) {

		// Retrieve taxonomy data
		$taxonomy_obj = get_taxonomy( $taxonomy_slug );
		$taxonomy_name = $taxonomy_obj->labels->name;

		// Retrieve taxonomy terms
		$terms = get_terms( $taxonomy_slug );

		// Display filter HTML
		echo "<select name='{$taxonomy_slug}' id='{$taxonomy_slug}' class='postform'>";
		echo '<option value="">' . sprintf( esc_html__( 'Show All %s', 'text_domain' ), $taxonomy_name ) . '</option>';
		foreach ( $terms as $term ) {
			printf(
				'<option value="%1$s" %2$s>%3$s (%4$s)</option>',
				$term->slug,
				( ( isset( $_GET[$taxonomy_slug] ) && ( $_GET[$taxonomy_slug] == $term->slug ) ) ? ' selected="selected"' : '' ),
				$term->name,
				$term->count
			);
		}
		echo '</select>';
	}

}
add_action( 'restrict_manage_posts', 'filter_customcpt_by_taxonomies' , 10, 2);

?>