v3

add_action( 'add_meta_boxes', 'wpse_143600_add_box' );
add_action( 'save_post', 'wpse_143600_save_box' );

//create the metabox
function wpse_143600_add_box() {
    add_meta_box(
        'related_testimonial',
        __( 'community', 'wpse_143600_translation' ),
        'wpse_143600_testimonial_box',
        'testimonial',
  'normal'
    );
}

//build the box
function wpse_143600_testimonial_box($post) {

    wp_nonce_field( basename( __FILE__ ), 'wpse_143600_nonce' );

    $wpse_143600_stored_meta = get_post_meta( $post->ID );

    $testimonialArgs = array(
  'post_type' => 'community',
  'post_status' => 'publish',
  'numberposts' => -1
    );

    $testimonials = get_posts($testimonialArgs);

    if($testimonials): ?>
    <p>
      <label for="meta-select" class="wpse_143600-row-title"><?php _e( 'Example Select Input', 'wpse_143600_translation' )?></label>
      <select name="meta-select" id="meta-select">
        <option value="NULL">Please choose a testimonial…</option>
    <?php foreach($testimonials as $testimonial): ?>
        <option value="<?php echo $testimonial->ID; ?>" <?php if ( isset ( $wpse_143600_stored_meta['meta-select'] ) ) selected( $wpse_143600_stored_meta['meta-select'][0], $testimonial->ID ); ?>><?php echo $testimonial->post_title; ?></option>
    <?php endforeach; ?>
      </select>
    </p>
    <?php
    else:
    ?>
  <p>There are no testimonials - please save this post, and write some testimonials. You'll then be able to choose a testimonial for this location.</p>
    <?php
    endif;
}

//save the box
function wpse_143600_save_box( $post_id ) {
    $is_autosave = wp_is_post_autosave( $post_id );
    $is_revision = wp_is_post_revision( $post_id );
    $is_valid_nonce = ( isset( $_POST[ 'wpse_143600_nonce' ] ) && wp_verify_nonce( $_POST[ 'wpse_143600_nonce' ], basename( __FILE__ ) ) ) ? 'true' : 'false';

    // Exits script depending on save status
    if ( $is_autosave || $is_revision || !$is_valid_nonce ) {
        return;
    }
    // Checks for input and saves if needed
    if( isset( $_POST[ 'meta-select' ] ) ) {
      update_post_meta( $post_id, 'meta-select', $_POST[ 'meta-select' ] );
    }
}