os.mkdir(support.TESTFN_UNDECODABLE) fails on Mono

For example, the following fails on Mono Linux:
```Python
os.mkdir(b'@test_16742_tmp\xe7w\xf0') 
```
with the following error:
> ValueError: Illegal byte sequence encounted in the input.

This causes failures in `test_genericpath.test_nonascii_abspath` which is run by `test_macpath`, `test_ntpath`, `test_posixpath`.