This supports -valgrind as with runner.go. It also works on Windows and
provides a place for implementing Chrome infra's JSON test output format in the
future, as well as whatever magic may be needed for Android.
Change-Id: I26eb68053f95e825561a142dbcdc4fbd84e3687d
Reviewed-on: https://boringssl-review.googlesource.com/3411
Reviewed-by: Adam Langley <agl@google.com>
