Passing --quiet makes valgrind only print out errors, so we don't need
to suppress things. Combine that with checking valgrind's dedicated exit
code so we notice errors that happen before the "---DONE---" marker.

This makes that marker unnecessary for valgrind. all_tests.go was not
sensitive to this, but still would do well to have valgrind be silent.

Change-Id: I841edf7de87081137e38990e647e989fd7567295
Reviewed-on: https://boringssl-review.googlesource.com/11128
Reviewed-by: Adam Langley <agl@google.com>