Fix the following on Windows:
* `spack load --list` (this printed 0 packages even if packages were
loaded)
* `spack unload <package>` (this said that the package is not loaded
even if it was)
Update unit tests for `spack load` to also run on Windows (specifically
for ".bat"). This involved refactoring a few tests to parameterize
based on whether the unit tests are being run on a Windows system
(and to account for batch syntax).
4c7d18a772