TST: fix X/Y Resolution didn't roundtrip in test_file_libtiff

Fixes false test failures on Windows with libtiff 4.0.3:
```
running test_file_libtiff ...
Tests\test_file_libtiff.py:127: assert_equal(original[tag], value, "%s didn't roundtrip" % tag) failed:
- XResolution didn't roundtrip
Tests\test_file_libtiff.py:127: assert_equal(original[tag], value, "%s didn't roundtrip" % tag) failed:
- YResolution didn't roundtrip
Tests\test_file_libtiff.py:131: assert_equal(value, reloaded[tag], "%s didn't roundtrip" % tag) failed:
- XResolution didn't roundtrip
Tests\test_file_libtiff.py:131: assert_equal(value, reloaded[tag], "%s didn't roundtrip" % tag) failed:
- YResolution didn't roundtrip
```

Alternatively, the `XResolution` and `YResolution` tags could simply be excluded from roundtrip testing at https://github.com/python-imaging/Pillow/blob/master/Tests/test_file_libtiff.py#L123.