Remove tests that include tcPDF and DomPDF libraries when running against PHP8, because neither library is yet PHP8-ready

tests/PhpSpreadsheetTests/Helper/SampleTest.php

@@ -31,6 +31,16 @@ class SampleTest extends TestCase
             'Chart/32_Chart_read_write_PDF.php', // Unfortunately JpGraph is not up to date for latest PHP and raise many warnings
             'Chart/32_Chart_read_write_HTML.php', // idem
         ];
+        // TCPDF and DomPDF libraries don't support PHP8 yet
+        if (\PHP_VERSION_ID >= 80000) {
+            $skipped = array_merge(
+                $skipped,
+                [
+                    'Pdf/21_Pdf_Domdf.php',
+                    'Pdf/21_Pdf_TCPDF.php',
+                ]
+            );
+        }
 
         // Unfortunately some tests are too long be ran with code-coverage
         // analysis on Travis, so we need to exclude them