Don't use abort_on_error for lsan darwin test suite

Summary:
This option is disabled by our other test suites, and will cause
failures when unit tests abort instead of failing with an error code.
Will also prevent the test suite from being too slow.

Reviewers: kubamracek, alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32129

llvm-svn: 300593