WP Auto-submit select using wp_dropdown_categories. These usually go in the same file but i separated them for (my) clarity.

loop.php

<?php 
$category_id = htmlspecialchars($_GET['cat']);
if( !empty($category_id) ){
$args = array(
	'posts_per_page'   => -1,
	'post_type'        => 'testimonials',
	'orderby'          => 'menu_order',
	'order' 		   => 'ASC',
	'post_status'      => 'publish',
		'tax_query' => array(
			array(
				'taxonomy' => 'review-types',
				'field'    => 'id',
				'terms'    => $category_id,
			),
		),
	);
}
else {
$args = array(
	'posts_per_page'   => -1,
	'post_type'        => 'testimonials',
	'orderby'          => 'menu_order',
	'order' 		   => 'ASC',
	'post_status'      => 'publish'
);
}

$reviews = new WP_Query( $args );

if ( $reviews->have_posts() ) : ?>


<!-- the loop -->
<?php while ( $reviews->have_posts() ) : $reviews->the_post(); ?>
	   <?php $image_check = pk_get_featured_image( $reviews->ID, 'circle' ); 
	   if( !empty( $image_check ) ):
	?>
	      <div class="row">
	         <div class="col-xs-12 col-sm-3">	
	         <img src="<?php echo $image_check ?>" alt="Reviews" class="img-responsive img-circle">
	      </div>
	<?php endif; ?>
	<?php $terms = get_the_terms($reviews->ID, 'review-types'); ?>
	
	<?php if( !empty( $image_check ) ): ?>
	   <div class="col-xs-12 col-sm-9">
	<?php else: ?>
	   <div class="review">
	<?php endif; ?>
	<div class="cats">
		<?php 
			if( !empty( $terms ) ):
				foreach( $terms as $check => $te ):
					$size = sizeof($terms);
					echo $te->name;
					if( $check + 1 != $size ){ echo ", "; }
				endforeach; 
			endif;
		?>
	</div>
	<h3><?php the_title(); ?></h3>
			<div class="excerpt">
				<?php the_content(); ?>
			</div>
		</div>
	<?php if( !empty( $image_check ) ): ?>
		</div> <!-- /row -->
		<div class="gray-border"></div>
	<?php endif; ?>
<?php endwhile; ?>
<!-- end of the loop -->

select.php

<label>Filter By Category: </label>
<form id="category-select" class="category-select" action="" method="GET">
	<?php
		$cat = '';
  		if( isset($_GET['cat']) ){  $cat = htmlspecialchars( $_GET['cat'] ); }
  		$tag ='';

		$args = array(
			'taxonomy'           => 'review-types',
			'show_option_all' => __( 'View All' ),
			'show_count'       => 0,
			'selected'         => $cat,
			'orderby'          => 'name',
			'echo'             => 0,
		);
		?>

		<?php $select  = wp_dropdown_categories( $args ); ?>
		<?php $replace = "<select$1 onchange='return this.form.submit()'>"; ?>
		<?php $select  = preg_replace( '#<select([^>]*)>#', $replace, $select ); ?>
		<div class=" custom-select">
			<?php echo $select; ?>
		</div>

		<noscript>
			<input type="submit" value="View" />
		</noscript>
</form>