test(perf): time the fastest batch so load can't fail the gate

Tests/CommandKitTests/PerformanceTests.swift

@@ -7,11 +7,18 @@ import Testing
 /// measures, so they catch an accidental quadratic loop without flaking on a busy CI runner.
 @Suite(.serialized)
 struct PerformanceTests {
+    /// The mean of the fastest of five batches. Other work on the machine can only slow a batch down, never
+    /// speed it up, so the fastest batch is the closest to the code's own cost. An average over everything
+    /// let one busy moment fail the gate on a loaded Mac.
     func averageMicroseconds(_ iterations: Int, _ body: () -> Void) -> Double {
         body()
-        let start = DispatchTime.now().uptimeNanoseconds
-        for _ in 0..<iterations { body() }
-        return Double(DispatchTime.now().uptimeNanoseconds - start) / Double(iterations) / 1000
+        var fastest = UInt64.max
+        for _ in 0..<5 {
+            let start = DispatchTime.now().uptimeNanoseconds
+            for _ in 0..<iterations { body() }
+            fastest = min(fastest, DispatchTime.now().uptimeNanoseconds - start)
+        }
+        return Double(fastest) / Double(iterations) / 1000
     }
 
     /// Two thousand plausible names: far more apps and panes than any Mac has.
@@ -32,7 +39,7 @@ struct PerformanceTests {
     func rankingTheWholeCatalogTakesWellUnderAFrame(query: String) {
         let catalog = Self.catalog
         let now = Date()
-        let us = averageMicroseconds(20) {
+        let us = averageMicroseconds(4) {
             _ = Ranker.rank(catalog, query: query, usage: Usage(), now: now, limit: 8)
         }
         print("rank 2,000 for \"\(query)\": \(String(format: "%.0f", us)) µs")