testing: Add Benchmark B struct stub

This struct allows test files containing basic benchmarks to compile
and run, but will not run the benchmarks themselves.

For #491

src/testing/benchmark.go

@@ -0,0 +1,10 @@
+package testing
+
+// B is a type passed to Benchmark functions to manage benchmark timing and to
+// specify the number of iterations to run.
+//
+// TODO: Implement benchmarks. This struct allows test files containing
+// benchmarks to compile and run, but will not run the benchmarks themselves.
+type B struct {
+	N int
+}