Fix path completion test case added in rL335905 on Windows

Summary:
The test fails because we don't rewrite the slash behind `foo` to the OS specific
separator (as the completion API doesn't support this kind of rewriting). However,
we assume that this part of the string is rewritten in the test case, which broke
on Windows.

Reviewers: stella.stamenova

Reviewed By: stella.stamenova

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D48746

llvm-svn: 335934