[mypyc] Show traceback when emitfunc unit test fails (#17262)

This makes debugging test failures easier.

mypy/test/helpers.py

@@ -104,7 +104,9 @@ def render_diff_range(
             output.write("\n")
 
 
-def assert_string_arrays_equal(expected: list[str], actual: list[str], msg: str) -> None:
+def assert_string_arrays_equal(
+    expected: list[str], actual: list[str], msg: str, *, traceback: bool = False
+) -> None:
     """Assert that two string arrays are equal.
 
     Display any differences in a human-readable form.
@@ -136,7 +138,7 @@ def assert_string_arrays_equal(expected: list[str], actual: list[str], msg: str)
             "Update the test output using --update-data -n0 "
             "(you can additionally use the -k selector to update only specific tests)\n"
         )
-        pytest.fail(msg, pytrace=False)
+        pytest.fail(msg, pytrace=traceback)
 
 
 def assert_module_equivalence(name: str, expected: Iterable[str], actual: Iterable[str]) -> None:

mypyc/test/test_emitfunc.py

@@ -841,7 +841,9 @@ def assert_emit(
         else:
             expected_lines = expected.rstrip().split("\n")
         expected_lines = [line.strip(" ") for line in expected_lines]
-        assert_string_arrays_equal(expected_lines, actual_lines, msg="Generated code unexpected")
+        assert_string_arrays_equal(
+            expected_lines, actual_lines, msg="Generated code unexpected", traceback=True
+        )
         if skip_next:
             assert visitor.op_index == 1
         else: