Fixed bug with nested Run blocks

[brodie-roberts-ix]

I've been using your package for a project I'm working on at work. I found a bug when nesting Run blocks, which can be reproduced like so:

func BenchmarkABC(tb *testing.B) {
	b := pbench.New(tb)
	b.ReportPercentile(0.99)

	b.Run("D", func(b *pbench.B) {
		b.Run("E", func(b *pbench.B) {
			b.RunParallel(func(pb *pbench.PB) {
				for pb.Next() {
					fmt.Print(".")
					time.Sleep(100 * time.Millisecond)
				}
			})
		})
	})
}

This one-line change should fix the issue!

[brodie-roberts-ix]

Upon using this further I've found a second panic case once two such Run blocks are put consecutively. The panic is on line 91 of pbench.go and triggers when Run "F" returns. I can't figure this one out easily, so I'll just have to fall back to only nesting Runs 1 deep.

func BenchmarkABC(tb *testing.B) {
	b := pbench.New(tb)
	b.ReportPercentile(0.99)

	b.Run("D", func(b *pbench.B) {
		b.Run("E", func(b *pbench.B) {
			b.RunParallel(func(pb *pbench.PB) {
				for pb.Next() {
					fmt.Print(".")
					time.Sleep(100 * time.Millisecond)
				}
			})
		})
	})

	b.Run("F", func(b *pbench.B) {
		b.Run("G", func(b *pbench.B) {
			b.RunParallel(func(pb *pbench.PB) {
				for pb.Next() {
					fmt.Print(".")
					time.Sleep(100 * time.Millisecond)
				}
			})
		})
	}) // triggers here
}