testing: with -benchtime=1x, run the benchmark loop exactly once

Like with -benchtime=1ns, if we find that the "discovery" round (run1)
has already crossed the -benchtime threshold, we skip running more
iterations.

Fixes #32051

Change-Id: I76aaef2ba521ba8ad6bbde2b14977e191aada5e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/331089
Trust: Caleb Spare <cespare@gmail.com>
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>

src/cmd/go/testdata/script/test_benchmark_1x.txt

@@ -0,0 +1,37 @@
+# Test that -benchtime 1x only runs a total of 1 loop iteration.
+# See golang.org/issue/32051.
+
+go test -run ^$ -bench . -benchtime 1x
+
+-- go.mod --
+module bench
+
+go 1.16
+-- x_test.go --
+package bench
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+var called = false
+
+func TestMain(m *testing.M) {
+	m.Run()
+	if !called {
+		fmt.Println("benchmark never called")
+		os.Exit(1)
+	}
+}
+
+func Benchmark(b *testing.B) {
+	if b.N > 1 {
+		b.Fatalf("called with b.N=%d; want b.N=1 only", b.N)
+	}
+	if called {
+		b.Fatal("called twice")
+	}
+	called = true
+}

src/testing/benchmark.go

@@ -299,7 +299,12 @@ func (b *B) launch() {
 
 	// Run the benchmark for at least the specified amount of time.
 	if b.benchTime.n > 0 {
+		// We already ran a single iteration in run1.
+		// If -benchtime=1x was requested, use that result.
+		// See https://golang.org/issue/32051.
+		if b.benchTime.n > 1 {
 			b.runN(b.benchTime.n)
+		}
 	} else {
 		d := b.benchTime.d
 		for n := int64(1); !b.failed && b.duration < d && n < 1e9; {