[libFuzzer] Use separate test directory for each config

Previously, check-all failed many tests for me.  It was running the
X86_64DefaultLinuxConfig, X86_64LibcxxLinuxConfig, and
X86_64StaticLibcxxLinuxConfig configs out of
llvm-build/projects/compiler-rt/test/fuzzer.  Now, it runs them out of
separate subdirectories there, and most tests pass.

Reviewed By: morehouse, george.karpenkov

Differential Revision: https://reviews.llvm.org/D49249

llvm-svn: 337521