ASan: add testcase for backtrace interceptor

It is a known, longstanding issue that some ASan interceptors
may write to freed memory, causing corruption
(https://github.com/google/sanitizers/issues/321). This patch
adds a testcase for the backtrace interceptor (one of the
known cases).

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D150491