[lldb/Test] Try to appease the Windows bot

In TestConvenienceVariables I changed %t from a file to a directory.
This tripped up mkdir which can't deal with an existing file at the
given location. In order to solve this issue on the bots I added an
`rm -rf %t` statement, but now the Windows bot complains that "This
function is not supported on this system".

If you never ran the test suite wit this temporary workaround, the test
might fail. If this happens please remove what %t expands to in the lit
output and rerun the test.

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