Fix test-data diffs not to produce traceback (#14549)

If the cause for failure is a diff, the traceback isn't helpful and in
fact we've been trying to suppress it with `__tracebackhide__` already.

Author: ikonst

mypy/test/data.py

@@ -359,12 +359,14 @@ def reportinfo(self) -> tuple[str, int, str]:
         return self.file, self.line, self.name
 
     def repr_failure(self, excinfo: Any, style: Any | None = None) -> str:
-        if excinfo.errisinstance(SystemExit):
+        if isinstance(excinfo.value, SystemExit):
             # We assume that before doing exit() (which raises SystemExit) we've printed
             # enough context about what happened so that a stack trace is not useful.
             # In particular, uncaught exceptions during semantic analysis or type checking
             # call exit() and they already print out a stack trace.
             excrepr = excinfo.exconly()
+        elif isinstance(excinfo.value, pytest.fail.Exception) and not excinfo.value.pytrace:
+            excrepr = excinfo.exconly()
         else:
             self.parent._prunetraceback(excinfo)
             excrepr = excinfo.getrepr(style="short")

mypy/test/helpers.py

@@ -51,8 +51,6 @@ def assert_string_arrays_equal(expected: list[str], actual: list[str], msg: str)
 
     Display any differences in a human-readable form.
     """
-    __tracebackhide__ = True
-
     actual = clean_up(actual)
     actual = [line.replace("can't", "cannot") for line in actual]
     expected = [line.replace("can't", "cannot") for line in expected]
@@ -117,7 +115,7 @@ def assert_string_arrays_equal(expected: list[str], actual: list[str], msg: str)
             # long lines.
             show_align_message(expected[first_diff], actual[first_diff])
 
-        raise AssertionError(msg)
+        pytest.fail(msg, pytrace=False)
 
 
 def assert_module_equivalence(name: str, expected: Iterable[str], actual: Iterable[str]) -> None: