Add future wait benchmark to catch memory leaks (#2931)

### Motivation:

In the past we introduced a memory leak around the creation of and
waiting on futures - we should protect against leaks in this fundamental
operation.

### Modifications:

Add a new benchmark which would have failed with the previous bug

### Result:

Regression protection for this type of memory leak.

Benchmarks/Benchmarks/NIOCoreBenchmarks/Benchmarks.swift

@@ -21,6 +21,11 @@ let benchmarks = {
         .mallocCountTotal
     ]
 
+    let leakMetrics: [BenchmarkMetric] = [
+        .mallocCountTotal,
+        .memoryLeaked,
+    ]
+
     Benchmark(
         "NIOAsyncChannel.init",
         configuration: .init(
@@ -46,4 +51,28 @@ let benchmarks = {
             blackHole(asyncChanel)
         }
     }
+
+    Benchmark(
+        "WaitOnPromise",
+        configuration: .init(
+            metrics: leakMetrics,
+            scalingFactor: .kilo,
+            maxDuration: .seconds(10_000_000),
+            maxIterations: 10_000  // need 10k to get a signal
+        )
+    ) { benchmark in
+        // Elide the cost of the 'EmbeddedEventLoop'.
+        let el = EmbeddedEventLoop()
+
+        benchmark.startMeasurement()
+        defer {
+            benchmark.stopMeasurement()
+        }
+
+        for _ in 0..<benchmark.scaledIterations.count {
+            let p = el.makePromise(of: Int.self)
+            p.succeed(0)
+            do { _ = try! p.futureResult.wait() }
+        }
+    }
 }

Benchmarks/Thresholds/5.10/NIOCoreBenchmarks.WaitOnPromise.p90.json

@@ -0,0 +1,4 @@
+{
+  "mallocCountTotal" : 6000,
+  "memoryLeaked" : 0
+}

Benchmarks/Thresholds/5.8/NIOCoreBenchmarks.WaitOnPromise.p90.json

@@ -0,0 +1,4 @@
+{
+  "mallocCountTotal" : 6000,
+  "memoryLeaked" : 0
+}

Benchmarks/Thresholds/5.9/NIOCoreBenchmarks.WaitOnPromise.p90.json

@@ -0,0 +1,4 @@
+{
+  "mallocCountTotal" : 6000,
+  "memoryLeaked" : 0
+}