[lldb][test][NFC] Narrow scope of `import pexpect`

We do not run `pexpect` based tests on Windows, but there are still cases where those tests run `import pexpect` outside of the scope where the test is skipped. By moving the import statement to a different scope, those tests can run even when `pexpect` truly isn't installed.

Tangentially related: TestSTTYBeforeAndAfter.py is using a manual `@expectedFailureAll` for windows instead of the common `@skipIfWindows`. If `pexepect` is generally expected to not be available, we should not bother running the test at all.