Centralize the handling of attach permissions on linux in tests

Summary:
On linux we need the process to give us special permissions before we can attach to it.
Previously, the code for this was copied into every file that needed it. This moves the code to a
central place to reduce code duplication.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D15992

llvm-svn: 257319