String.Convert To Lowercase (#1735): Use our custom util that avoids IronPython bug
IronPython atest workarounds.

- Made non-ASCII tests that cannot work on IronPython non-critical.
- Longer timeouts for tests that can be sloooow.
Fixed variable recommendation test on Windows (#1804).

Need to use upper case environment variable names in tests because
Robot's `utils.get_env_vars()` uses them on Windows by default to
avoid differences between Python interpreters.

Also changed the test slightly. Earlier it failed already when
resolving the internal environment variable and, for example,
${normal_var} was not evaluated at all. @guykisel, was that on
purpose?