Assert the exact round-robin order in the starvation test

Flush each stream individually so the order streams enter the rotation is
fixed rather than dependent on Set iteration order, letting the test assert
the served sequence exactly.

Author: Nishad Deokar

Tests/NIOHTTP2Tests/OutboundFlowControlBufferTests.swift

@@ -402,15 +402,11 @@ class OutboundFlowControlBufferTests: XCTestCase {
             self.buffer.streamCreated(streamID, initialWindowSize: 15)
             let frame = self.createDataFrame(streamID, byteBufferSize: 15)
             XCTAssertNoThrow(try self.buffer.processOutboundFrame(frame, promise: nil).assertNothing())
+            self.buffer.flushReceived()
         }
-        self.buffer.flushReceived()
-
-        let served = self.receivedFrames().map { $0.streamID }
 
         // Every stream must be served once per round so that none can be starved.
-        let firstRound = Array(served.prefix(streamIDs.count))
-        XCTAssertEqual(Set(firstRound), Set(streamIDs))
-        XCTAssertEqual(served, firstRound + firstRound + firstRound)
+        XCTAssertEqual(self.receivedFrames().map { $0.streamID }, [1, 3, 5, 1, 3, 5, 1, 3, 5])
     }
 
     func testRejectsPrioritySelfDependency() {