Add lldb-mi/lldb-server test folders to PATH before in dotest.py

Summary:
This patch allows not specify search path in each lldb-mi test. It makes tests easier.

This fix was requested by vharron.

All test pass on OS X.

Reviewers: vharron, clayborg

Subscribers: lldb-commits, vharron

Differential Revision: http://reviews.llvm.org/D8207

llvm-svn: 232019