Don't source local .lldbinit in the test suite

Summary:
As suggested by Pavel, we shouldn't let our tests parse the local .lldbinit to prevent random test failures
due to changed settings.

Fixes Minidump/Windows/Sigsegv/sigsegv.test (and probably others too).

Reviewers: labath, serge-sans-paille

Reviewed By: labath

Subscribers: abidh, lldb-commits, zturner

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D58235

llvm-svn: 354038