[lldb/test] Use @skipIfWindows for PExpectTest

Annotating `PExpectTest` with `@skipIfWindows` instead of marking it as an empty class will make the test runner recognize it as a test class, which should allow me to reland adb5c23f.

I don't have a windows machine to verify this works, but I did some tests using `@skipIfLinux` and they all worked as expected. In case the `pexpect` import is not at all available on windows, I moved it to within the method where it's used.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D86745