[lldb][test] Enforce `pexpect` system availability by default (#84270)

This switches the default of `LLDB_TEST_USE_VENDOR_PACKAGES` from `ON`
to `OFF` in preparation for eventually deleting it. All known LLDB
buildbots have this package installed, so flipping the default will
uncover any other users.

If this breaks anything, the preferred fix is to install `pexpect` on
the host system. The second fix is to build with cmake option
`-DLLDB_TEST_USE_VENDOR_PACKAGES=ON` as a temporary measure until
`pexpect` can be installed. If neither of those work, reverting this
patch is OK.