Add BenchmarkCreateGoroutinesLeftOpen

This benchmark tests the performance of the runtime when opening thousands of goroutines simultaneously, and leaving them open. It may effectively be testing the internal allocation routines, but there seems to be a [substantial performance regression](https://github.com/nickstenning/autobench/blob/965fba8f965b7a26d0d50bb672e996c5866e8b32/darwin-amd64-runtime-112ea225794f.txt#L79) in 1.1.