[asan] remove one redundant malloc stress test, unify the usage of ASAN_LOW_MEMORY macro in tests, slightly reduce test memory usage (all to make 32-bit runs consume less RAM)

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@177069 91177308-0d34-0410-b5e6-96231b3b80d8