test-unsetenv needs to be re-enabled once the underlying problem is fixed

[dougburns]

test-unsetenv has guard code to prevent it running, as an issue in the underlying getenv/unsetenv implementations causes the test to loop.
This needs to be reverted once the underlying problem is fixed.
See #12 for a description of the problem and zopencommunity/gettextport#15 for the equivalent issue in gettextxport.