Test BenchmarkHttpClientTest does not reset resource manager or setupBenchmarkclient if client_ is not null

Description:

Inside our BenchmarkHttpClient Test Code, if the client_ member is null, we have an if condition that says to not reset resource manager and not to call setupBenchmarkClient. I'm not sure what benefit this provides. I think that it is more likely to lead to confusion as to why the benchmarkClient and resourceManager wasn't setup with the passed in parameters.

Relevant Links:
[BenchMarkHttpClientTest](https://github.com/envoyproxy/nighthawk/blob/master/test/benchmark_http_client_test.cc#L67)