[asan] Reduce flakiness of heavy recovery mode tests.

When we run halt_on_error-torture.cc with 10 threads and 20 iterations with halt_on_error=false:suppress_equal_pcs=false, we write 200 reports to 10.txt file and sometimes have collisions.
We have CHECK-COLLISION check that greps 'AddressSanitizer: nested bug in the same thread, aborting' message in 10.txt, but it doesn't contain this line.
If I don't redirect stderr > 10.txt 'AddressSanitizer: nested bug in the same thread, aborting' is printed to my screen as expected.
Same happens for halt_on_error_suppress_equal_pcs.cc and halt_on_error-torture.cc. This happens because of kernel bug: https://lkml.org/lkml/2014/2/17/324
Furtunately, we can fix these tests by implicitly setting O_APPEND for opened files (use >> instead of > for stderr redirection).

Differential Revision: https://reviews.llvm.org/D22921

llvm-svn: 277324