[lldb][test] Add `pexpect` category for tests that `import pexpect` (#84860)

Instead of directly annotating pexpect-based tests with
`@skipIfWindows`, we can tag them with a new `pexpect` category. We
still automatically skip windows behavior by adding `pexpect` to the
skip category list if the platform is windows, but also allow
non-Windows users to skip them by configuring cmake with
`-DLLDB_TEST_USER_ARGS=--skip-category=pexpect`

As a prerequisite, remove the restriction that `@add_test_categories`
can only apply to test cases, and we make the test runner look for
categories on both the class and the test method.