Test-suite - Show warning when ".fixed" file not found (#336)

This commit updates the `AbstractSniffUnitTest` class used for the sniff tests to show a non-build-failing warning when a test case file would lead to fixable errors/warnings, but there is no `.fixed` version available for the test case file to verify the fixes being made.

This is a preliminary step towards making missing `.fixed` files a test failing error in PHPCS 4.0.

Note: the warning will only show when tests are being run on PHPUnit 7.3.0 or higher.

Author: fredden

tests/Standards/AbstractSniffUnitTest.php

@@ -190,15 +190,17 @@ final public function testSniff()
 
                 // Check for a .fixed file to check for accuracy of fixes.
                 $fixedFile = $testFile.'.fixed';
+                $filename  = basename($testFile);
                 if (file_exists($fixedFile) === true) {
                     $diff = $phpcsFile->fixer->generateDiff($fixedFile);
                     if (trim($diff) !== '') {
-                        $filename          = basename($testFile);
                         $fixedFilename     = basename($fixedFile);
                         $failureMessages[] = "Fixed version of $filename does not match expected version in $fixedFilename; the diff is\n$diff";
                     }
+                } else if (is_callable([$this, 'addWarning']) === true) {
+                    $this->addWarning("Missing fixed version of $filename to verify the accuracy of fixes, while the sniff is making fixes against the test case file");
                 }
-            }
+            }//end if
 
             // Restore the config.
             $config->setSettings($oldConfig);