Fix pytest valgrind warnings

[wiredfool]

Potential fix for pytest-valgrind mark not defined for non-valgrind docker tests.

[hugovk]

Thanks! Confirmed fixed in a dummy commit: https://github.com/hugovk/Pillow/runs/2244123927?check_suite_focus=true

[hugovk]

Although it does detect a bunch of errors. Should these also fail the CI?

Valgrind has detected an error:

**304** 
**304** **********************************************************************
**304** Tests/test_image.py::TestImage::test_exif_jpeg
**304** **********************************************************************
**304** 
**304** **********************************************************************

XFAIL Tests/test_image_resample.py::TestCoreResampleAlphaCorrect::test_levels_rgba
  reason: Error, but valgrind clean, using xfail.
XFAIL Tests/test_image_resample.py::TestCoreResampleAlphaCorrect::test_levels_la
  reason: Error, but valgrind clean, using xfail.
XFAIL Tests/test_image_resample.py::TestCoreResampleBox::test_tiles
  reason: [VALGRIND ERROR]

Valgrind has detected an error:

**304** 
**304** **********************************************************************
**304** Tests/test_image_resample.py::TestCoreResampleBox::test_tiles
**304** **********************************************************************
==304== Use of uninitialised value of size 8
==304==    at 0x8B8BB87: ImagingResampleHorizontal_8bpc (Resample.c:346)
==304==    by 0x8B8C03B: ImagingResampleInner (Resample.c:664)
==304==    by 0x8B8C30A: ImagingResample (Resample.c:611)
==304==    by 0x8B812E5: _resize (_imaging.c:1812)
==304==    by 0x64BF47: method_vectorcall_VARARGS (descrobject.c:300)
==304==    by 0x4EB73C: _PyObject_Vectorcall (abstract.h:127)
==304==    by 0x4EB73C: call_function (ceval.c:4963)
==304==    by 0x4EB73C: _PyEval_EvalFrameDefault (ceval.c:3486)
==304==    by 0x4DF2EE: PyEval_EvalFrameEx (ceval.c:741)
==304==    by 0x4DFDFB: _PyEval_EvalCodeWithName (ceval.c:4298)
==304==    by 0x436C40: _PyFunction_Vectorcall (call.c:435)
==304==    by 0x4EB73C: _PyObject_Vectorcall (abstract.h:127)
==304==    by 0x4EB73C: call_function (ceval.c:4963)
==304==    by 0x4EB73C: _PyEval_EvalFrameDefault (ceval.c:3486)
==304==    by 0x4DF2EE: PyEval_EvalFrameEx (ceval.c:741)
==304==    by 0x43627B: function_code_fastcall (call.c:283)
==304== 
==304== Use of uninitialised value of size 8
==304==    at 0x8B8BB92: ImagingResampleHorizontal_8bpc (Resample.c:346)
==304==    by 0x8B8C03B: ImagingResampleInner (Resample.c:664)
==304==    by 0x8B8C30A: ImagingResample (Resample.c:611)
==304==    by 0x8B812E5: _resize (_imaging.c:1812)
==304==    by 0x64BF47: method_vectorcall_VARARGS (descrobject.c:300)
==304==    by 0x4EB73C: _PyObject_Vectorcall (abstract.h:127)
==304==    by 0x4EB73C: call_function (ceval.c:4963)
==304==    by 0x4EB73C: _PyEval_EvalFrameDefault (ceval.c:3486)
==304==    by 0x4DF2EE: PyEval_EvalFrameEx (ceval.c:741)
==304==    by 0x4DFDFB: _PyEval_EvalCodeWithName (ceval.c:4298)
==304==    by 0x436C40: _PyFunction_Vectorcall (call.c:435)
==304==    by 0x4EB73C: _PyObject_Vectorcall (abstract.h:127)
==304==    by 0x4EB73C: call_function (ceval.c:4963)
==304==    by 0x4EB73C: _PyEval_EvalFrameDefault (ceval.c:3486)
==304==    by 0x4DF2EE: PyEval_EvalFrameEx (ceval.c:741)
==304==    by 0x43627B: function_code_fastcall (call.c:283)
==304== 
==304== Use of uninitialised value of size 8
==304==    at 0x8B8BBA2: ImagingResampleHorizontal_8bpc (Resample.c:346)
==304==    by 0x8B8C03B: ImagingResampleInner (Resample.c:664)
==304==    by 0x8B8C30A: ImagingResample (Resample.c:611)
==304==    by 0x8B812E5: _resize (_imaging.c:1812)
==304==    by 0x64BF47: method_vectorcall_VARARGS (descrobject.c:300)
==304==    by 0x4EB73C: _PyObject_Vectorcall (abstract.h:127)
==304==    by 0x4EB73C: call_function (ceval.c:4963)
==304==    by 0x4EB73C: _PyEval_EvalFrameDefault (ceval.c:3486)
==304==    by 0x4DF2EE: PyEval_EvalFrameEx (ceval.c:741)
==304==    by 0x4DFDFB: _PyEval_EvalCodeWithName (ceval.c:4298)
==304==    by 0x436C40: _PyFunction_Vectorcall (call.c:435)
==304==    by 0x4EB73C: _PyObject_Vectorcall (abstract.h:127)
==304==    by 0x4EB73C: call_function (ceval.c:4963)
==304==    by 0x4EB73C: _PyEval_EvalFrameDefault (ceval.c:3486)
==304==    by 0x4DF2EE: PyEval_EvalFrameEx (ceval.c:741)
==304==    by 0x43627B: function_code_fastcall (call.c:283)
==304== 

[wiredfool]

There are a few XFAILed here because they're undefined behaviour and I haven't tracked them down yet. (some of which are deep in JPEG guts). The recent addition of the fuzzer test -- which runs all the images through open/load has retriggered some of them that are xfailed (I think) but I haven't had a chance to get to them.

My idea for now is to prevent additional items from making valgrind worse. Which I'm failing at a bit with the fuzzer test, but to be fair, that's increased testing, not increased code to be tested.