Improve Identification of Samples in Coverage Report (#2153)

The Phpunit coverage report currently contains bullet items like `PhpOffice\PhpSpreadsheetTests\Helper\SampleTest\testSample with data set "49"`. This extremely simple change takes advantage of Phpunit's ability to accept an array with keys which are either strings or integers, by using the sample filenames as the array keys rather than sequential but otherwise meaningless integers (e.g. `49` in the earlier cited item). The bullet item will now read `PhpOffice\PhpSpreadsheetTests\Helper\SampleTest\testSample with data set "Basic/38_Clone_worksheet.php"`.

tests/PhpSpreadsheetTests/Helper/SampleTest.php

@@ -63,7 +63,7 @@ class SampleTest extends TestCase
 //                }
                 if (!in_array($sample, $skipped)) {
                     $file = 'samples/' . $sample;
-                    $result[] = [$file];
+                    $result[$sample] = [$file];
                 }
             }
         }