flaky test: `testConnectionPoolGrowsToMaxConcurrentConnections`

There are a couple of issues with testConnectionPoolGrowsToMaxConcurrentConnections:

• The maxConnections variable isn't passed down to the ConnectionPool constructor. The expected value 8 happens to match the default, but shouldn't this test use a non-default value, to check that the parameter works?

• It checks:

  XCTAssertEqual(httpBin.createdConnections, maxConnections)
  

  But actually, httpBin.createdConnections > maxConnections is also valid. The ConnectionsCountHandler doesn't actually measure the maximum number of active connections it saw -- just the total number that were created. If I have a couple of Xcode projects open, some Safari windows each with lots of tabs, I can see 10 created connections (even though when logging, I see that the maximum concurrent connections parameter was always respected).

Start with testConnectionPoolGrowsToMaxConcurrentConnections in Tests/AsyncHTTPClientTests/HTTPConnectionPoolTests.swift and inspect ConnectionsCountHandler in Tests/AsyncHTTPClientTests/HTTPClientTestUtils.swift. Run the test and verify it passes a non-default maxConnections value, measures the maximum concurrent connections rather than only total connections created, and accepts valid connection recreation.