Replace deprecated function format_string() by new FormattableMarkup

Problem/Motivation

Change records: https://www.drupal.org/node/2302363

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  Line   src/Commands/LinkCheckerCommands.php                                       
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         Class Drush\Commands\DrushCommands not found and could not be autoloaded.  
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  Line   src/Tests/LinkCheckerInterfaceTest.php                                   
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  144    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
  147    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
  194    Call to deprecated function db_query():                                  
         as of Drupal 8.0.x, will be removed in Drupal 9.0.0. Instead, get        
         a database connection injected into your service from the container and  
         call query() on it. For example,                                         
  208    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
  211    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
  240    Call to deprecated function db_update():                                 
         as of Drupal 8.0.x, will be removed in Drupal 9.0.0. Instead, get        
         a database connection injected into your service from the container and  
         call update() on it. For example,                                        
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 ------ ------------------------------------------------------------------------- 
  Line   src/Tests/LinkCheckerLinkExtractionTest.php                              
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  327    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
  330    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
  339    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
  378    Call to deprecated function db_query():                                  
         as of Drupal 8.0.x, will be removed in Drupal 9.0.0. Instead, get        
         a database connection injected into your service from the container and  
         call query() on it. For example,                                         
  392    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
  395    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
  402    Call to deprecated function format_string():                             
         in Drupal 8.0.0, will be removed before Drupal 9.0.0.                    
         Use \Drupal\Component\Render\FormattableMarkup.                          
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Proposed resolution

Replace deprecated function format_string() by new FormattableMarkup