Supports regression test in fuzzing_launcher (#56)

Supports regression test in fuzzing_launcher

Adds string_flag engine to decide the fuzzing_launcher behavior.

Adds engine to launcher and adds is_regression to fuzzing_launcher

Constructs all launcher args in a list

Signed-off-by: tengpeng <tengpeng.li2020@gmail.com>

	modified:   .bazelrc
	modified:   WORKSPACE
	modified:   fuzzing/BUILD
	modified:   fuzzing/common.bzl
	modified:   fuzzing/repositories.bzl
	modified:   fuzzing/tools/launcher.py