Trim end of name, keeping ext

[radarhere]

Suggestion for python-pillow#4424 - trims the name of a long filename, while keeping the extension.

In the test I've added,
name_limit_testname_limit_testname_limit_testname_limit_testname_limit_testname_limit_testname_limit_test.im
is trimmed to
name_limit_testname_limit_testname_limit_testname_limit_testname_limit_testname_limit_tes.im

Since the name field isn't accessible from Pillow, I saved an IM file with a long name, renamed it to something shorter just for neatness, and then use that file as the reference.

[hugovk]

Thank you!