Test/EnvironmentTest.php
getInstance will not throw an exception if all tests are running.
make set instance null possible and check for type magento_environtment.

implement skipped Varien_Date tests.
is it correct that %s property will be ignored? the test failed yet.

Implementation of ImageMagickTest will not work under windows because touch is an unix command.
check for environtment and skip these tests.

you should think about using of vfsStream for file system tests.