Fix Name field length when saving IM images

[hugovk]

For #4193.

Converting Tests/test_file_im.py to use pytest surfaced a bug in saving IM format files.

When roundtripping the file, it failed opening the saved file, at this check:

Pillow/src/PIL/ImImagePlugin.py

Lines 155 to 156 in 29fee8f

	if len(s) > 100:
	raise SyntaxError("not an IM file")

It's falling over the Name: field, which is saved like this:

Pillow/src/PIL/ImImagePlugin.py

Lines 349 to 350 in 29fee8f

	if filename:
	fp.write(("Name: %s\r\n" % filename).encode("ascii"))

The difference is the temp path+filename under the limit with PillowTestCase.tempfile:

Name: /var/folders/kt/j77sf4_n6fnbx6pg199rbx700000gn/T/tempx72ebkj5.im 

And over the limit with pytest's tmp_path:

Name: /private/var/folders/kt/j77sf4_n6fnbx6pg199rbx700000gn/T/pytest-of-hugo/pytest-126/test_roundtrip0/temp.im 

This PR trims the path+filename to fit, and I suggest to keep the last part which includes the filename and extension, rather than cropping it off.

Name: olders/kt/j77sf4_n6fnbx6pg199rbx700000gn/T/pytest-of-hugo/pytest-126/test_roundtrip0/temp.im 

[radarhere]

My personal opinion, in the case of /private/var/folders/kt/j77sf4_n6fnbx6pg199rbx700000gn/T/pytest-of-hugo/pytest-126/test_roundtrip0/temp.im, I find

Name: olders/kt/j77sf4_n6fnbx6pg199rbx700000gn/T/pytest-of-hugo/pytest-126/test_roundtrip0/temp.im 

to be slightly odd. I would feel better about just trimming it down to the base name in this situation, because I don't think I need to know the absolute path of an image's original location.

Name: temp.im 

[hugovk]

Sounds sensible, updated.

Of course, it's technically possible for a long basename to exceed the limit. This is less likely, do we fix it, or let it be until someone stumbles over it? There's an easy workaround.

I doubt the IM format gets much use, pretty much the only references I could find online for the format were in PIL/Pillow.