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

tests/tinygotest/main_test.go

@@ -14,3 +14,6 @@ func TestFail2(t *testing.T) {
 
 func TestPass(t *testing.T) {
 }
+
+func BenchmarkNotImplemented(b *testing.B) {
+}