SetEnvironmentVariableA("", value) crashes on NT4 -> removed.
The behavior of SetEnvironmentVariableA("", "") varies to much ->
removed.
Test the behavior of SetEnvironmentVariable{A,W}(name, "") taking into
account the differences between Win9x and NT.
Test the behavior of GetEnvironmentVariable{A,W}(name, buf, 0).
Fix the error code checks to take into account the differences between
Win9x, NT4 and XP.
Skip the unicode tests if the first call returns
ERROR_CALL_NOT_IMPLEMENTED.
Whenever a test fails, print sufficient information (typically return
code and GetLastError()!) to let the developer figure out what is
wrong.