Allow use of self.filecheck in LLDB tests (c.f self.expect)

Add a "filecheck" method to the LLDB test base. This allows test authors
to pattern match command output using FileCheck, making it possible to
write stricter tests than what `self.expect` allows.

For context (motivation, examples of stricter checking, etc), see the
lldb-dev thread: "Using FileCheck in lldb inline tests".

Differential Revision: https://reviews.llvm.org/D50751

llvm-svn: 342508