Fix the broken performance test binary (#2619)

Motivation:

The performance test binary was crashing ever since #2589 added the crash
on deinit flow. Crashes here are preventing us from using the performance
tester.

Modifications:

Correctly clean up the async writer.

Result:

The writer is cleaned up now.

Sources/NIOPerformanceTester/NIOAsyncWriterSingleWritesBenchmark.swift

@@ -44,7 +44,9 @@ final class NIOAsyncWriterSingleWritesBenchmark: AsyncBenchmark, @unchecked Send
     }
 
     func setUp() async throws {}
-    func tearDown() {}
+    func tearDown() {
+        self.writer.finish()
+    }
 
     func run() async throws -> Int {
         for i in 0..<self.iterations {