Don't automtically try to import pexpect in lldbpexpect.

Since pexpect doesn't exist on Windows, tests which are xfail'ed
are not being run at all because they are failing when the file
is imported due to the `import pexpect`.  This puts the import
behind a conditional and makes an empty base class in the case
where pexpect is not present.

llvm-svn: 258965