Minidump plugin: Fix flaky test

Summary:
One of the tests was flaky, because similarly to
https://reviews.llvm.org/D18697 (rL265391) - if there is a process running
which is with the same PID as in the core file, the minidump
core file debugging will fail, because we get some information from the
running process.
The fix is routing the ProcessInfo requests through the Process class
and overriding it in ProcessMinidump to return correct data.

Reviewers: labath

Subscribers: lldb-commits, beanz

Differential Revision: https://reviews.llvm.org/D26193

llvm-svn: 285698