Fix benchmark_http_client_test.cc and add test cases. (#368)
Fix benchmark_http_client_test.cc and add test cases. (#368)

Fix benchmark_http_client_test.cc which has a bug in function testBasicFunctionality().

For example, if we have a test fixture with testBasicFunctionality(max_pending=0,connection_limit=15,amount_of_request=10),
then EXPECT_FALSE(client_->tryStartRequest(f)) and
EXPECT_EQ(max_pending == 0 ? 1 : max_pending, inflight_response_count) will fail.

For test fixture with
testBasicFunctionality(max_pending=2,connection_limit=3,amount_of_request=10),
EXPECT_EQ(max_pending == 0 ? 1 : max_pending, inflight_response_count) will fail.

Added 2 test cases to verify the expected behavior fixed by this change.

Tested:
Unit test.
envoyproxy/nighthawk,envoyproxy/nighthawk,envoyproxy/nighthawk,envoyproxy/nighthawk