Make the runner tests a go “test”

This change makes the runner tests (in ssl/test/runner) act like a
normal Go test rather than being a Go binary. This better aligns with
some internal tools.

Thus, from this point onwards, one has to run the runner tests with `go
test` rather than `go run` or `go build && ./runner`.

This will break the bots.

Change-Id: Idd72c31e8e0c2b7ed9939dacd3b801dbd31710dd
Reviewed-on: https://boringssl-review.googlesource.com/6009
Reviewed-by: Matt Braithwaite <mab@google.com>
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/BUILDING.md b/BUILDING.md
index c75851f..65e88e8 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -91,6 +91,13 @@
     don't have steps for assembling the assembly language source files, so they
     currently cannot be used to build BoringSSL.
 
+# Running tests
+
+There are two sets of tests: the C/C++ tests and the blackbox tests. For former
+are built by Ninja and can be run from the top-level directory with `go run
+util/all_tests.go`. The latter have to be run separately by running `go test`
+from within `ssl/test/runner`.
+
 
  [1]: http://www.cmake.org/download/
  [2]: http://strawberryperl.com/