Rename spool dir entries to blackbox.XXXXX

This change makes it more natural for lava-android-test integration.  In
that mode l-a-t uses the on-device test results directory as the spool
directory and thus naturally creates additional entries there.

Change-Id: Iaec54d7df62d4b2172d0a247d85a826eb8071293
diff --git a/lava-blackbox b/lava-blackbox
index 6079a8f..9d25cc0 100755
--- a/lava-blackbox
+++ b/lava-blackbox
@@ -88,7 +88,7 @@
     fi
 
     # Generate a new unique directory where we will store each result
-    test_run_dir=$(mktemp -d -p $spool_dir run.XXXXX)
+    test_run_dir=$(mktemp -d -p $spool_dir blackbox.XXXXX)
     info "Test results will be saved to $test_run_dir"
 
     # Find all executables that look like _test and run them