On Windows, do not execute convert.exe without specifying path

`Convert.exe` is a system tool that converts file systems.
Windows users will need to set the `MAGICK_HOME` environment variable before running Pillow tests.

Fixes one test error:
```
======================================================================
ERROR: TestFilePalm.test_monochrome
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\Build\Pillow\Pillow-git\Tests\test_file_palm.py", line 39, in test_monochrome
    self.roundtrip(mode)
  File "D:\Build\Pillow\Pillow-git\Tests\test_file_palm.py", line 29, in roundtrip
    converted = self.open_withImagemagick(outfile)
  File "D:\Build\Pillow\Pillow-git\Tests\helper.py", line 160, in open_withImagemagick
    return Image.open(outfile)
  File "D:\Build\Pillow\Pillow-git\PIL\Image.py", line 2281, in open
    % (filename if filename else fp))
OSError: cannot identify image file 'C:\\Users\\gohlke\\AppData\\Local\\Temp\\tempe1e9r8fi.png'
```