[libFuzzer] Set default sanitizer options in fuzzer tests

Summary:
Set default `ASAN_OPTIONS` when running libFuzzer tests. This allows us
to remove special casing in code for Darwin where we usually pass
`abort_on_error=0` to override platform defaults for tests.

A previous commit changed the code to make the tests pass:
https://github.com/llvm/llvm-project/commit/7764a04af007eca68eafcf5caaea560ed05e35a9

Adapted a few tests to use `%env_asan_opts=` instead of directly setting
the environment variable.

rdar://problem/47515276

Reviewers: kcc, george.karpenkov

Differential Revision: https://reviews.llvm.org/D57465

llvm-svn: 352711