testing: improve B.Loop test

This moves the B.Loop test from package testing_test to package
testing, where it can check on more of the internals of the benchmark
state.

Updates #61515.

Change-Id: Ia32d7104526125c5e8a1e35dab7660008afcbf80
Reviewed-on: https://go-review.googlesource.com/c/go/+/635897
Auto-Submit: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

src/testing/loop_test.go

@@ -0,0 +1,51 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+func TestBenchmarkBLoop(t *T) {
+	var initialStart highPrecisionTime
+	var firstStart highPrecisionTime
+	var lastStart highPrecisionTime
+	runs := 0
+	iters := 0
+	finalBN := 0
+	bRet := Benchmark(func(b *B) {
+		initialStart = b.start
+		runs++
+		for b.Loop() {
+			if iters == 0 {
+				firstStart = b.start
+			}
+			lastStart = b.start
+			iters++
+		}
+		finalBN = b.N
+	})
+	// Verify that a b.Loop benchmark is invoked just once.
+	if runs != 1 {
+		t.Errorf("want runs == 1, got %d", runs)
+	}
+	// Verify that at least one iteration ran.
+	if iters == 0 {
+		t.Fatalf("no iterations ran")
+	}
+	// Verify that b.N, bRet.N, and the b.Loop() iteration count match.
+	if finalBN != iters || bRet.N != iters {
+		t.Errorf("benchmark iterations mismatch: %d loop iterations, final b.N=%d, bRet.N=%d", iters, finalBN, bRet.N)
+	}
+	// Make sure the benchmark ran for an appropriate amount of time.
+	if bRet.T < benchTime.d {
+		t.Fatalf("benchmark ran for %s, want >= %s", bRet.T, benchTime.d)
+	}
+	// Verify that the timer is reset on the first loop, and then left alone.
+	if firstStart == initialStart {
+		t.Errorf("b.Loop did not reset the timer")
+	}
+	if lastStart != firstStart {
+		t.Errorf("timer was reset during iteration")
+	}
+}
+
+// See also TestBenchmarkBLoop* in other files.