Update framework testing infrastructure and standards

- Add phpt test runner and suite for app functions
- Introduce testing workflow to AGENT.md
- Add tests for cache, content, context, helpers, hooks, plugins,
  rendering
- Mount tests directory in dev container

devel/tests/helpers/find_pdf_file.phpt

@@ -0,0 +1,28 @@
+--TEST--
+findPdfFile: finds first .pdf file in a directory
+--FILE--
+<?php
+require '/var/www/app/helpers.php';
+
+$dir = sys_get_temp_dir() . '/phpt_pdf_' . getmypid();
+mkdir($dir);
+
+// No PDF
+echo (findPdfFile($dir) ?? 'null') . "\n";
+
+// One PDF
+touch("$dir/document.pdf");
+echo findPdfFile($dir) . "\n";
+
+// Non-PDF files are ignored
+touch("$dir/image.jpg");
+touch("$dir/notes.txt");
+echo findPdfFile($dir) . "\n";
+
+array_map('unlink', glob("$dir/*"));
+rmdir($dir);
+?>
+--EXPECT--
+null
+document.pdf
+document.pdf