Slightly raise coverage

tests/XML/DOMDocumentFactoryTest.php

@@ -5,6 +5,7 @@
 namespace SimpleSAML\Test\XML;
 
 use DOMDocument;
+use Exception;
 use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\Attributes\Group;
 use PHPUnit\Framework\TestCase;
@@ -14,6 +15,8 @@
 use SimpleSAML\XML\Exception\SchemaViolationException;
 use SimpleSAML\XML\Exception\UnparseableXMLException;
 
+use function restore_error_handler;
+use function set_error_handler;
 use function strval;
 
 /**
@@ -135,4 +138,20 @@ public function testSchemaValidationWrongElementFails(): void
         $this->expectException(SchemaViolationException::class);
         DOMDocumentFactory::fromFile($file, $schemaFile);
     }
+
+
+    public function testSchemaValidationUnknownSchemaFileFails(): void
+    {
+        $file = 'tests/resources/xml/ssp_Chunk.xml';
+        $schemaFile = 'tests/resources/schemas/doesnotexist.xsd';
+
+        // Dirty trick to catch the warning emitted by PHP
+        set_error_handler(static function (int $errno, string $errstr): never {
+            restore_error_handler();
+            throw new Exception($errstr, $errno);
+        }, E_WARNING);
+
+        $this->expectExceptionMessage('Failed to locate the main schema resource at');
+        DOMDocumentFactory::fromFile($file, $schemaFile);
+    }
 }