Dump source files by lines, not by characters, when a test fails.

mjtest/test/tests.py

@@ -349,7 +349,7 @@ class ExtensibleTestResult(TestResult):
 
     def add_file(self, title: str, file_name: str, with_line_numbers: bool = True):
         with open(file_name, "r", errors="backslashreplace") as f:
-            file_content = os.linesep.join([line.rstrip() for line in f.read()])
+            file_content = os.linesep.join([line.rstrip() for line in f.readlines()])
         self.add_long_text(title, file_content, with_line_numbers)
 
     def succeeded(self):