Use `startIteration` in the tracing test

[inancgumus]

Why don't we use the startIteration helper and run tests in parallel as in here:

xk6-browser/tests/page_test.go

Lines 185 to 244 in b8d6418

	func TestPageEvaluateMapping(t *testing.T) {
	t.Parallel()
	tests := []struct {
	name string
	script string
	want any
	}{
	{
	name: "arrow",
	script: "() => 0",
	want: 0,
	},
	{
	name: "full_func",
	script: "function() {return 1}",
	want: 1,
	},
	{
	name: "arrow_func_no_return",
	script: "() => {2}",
	want: sobek.Null(),
	},
	{
	name: "full_func_no_return",
	script: "function() {3}",
	want: sobek.Null(),
	},
	{
	name: "async_func",
	script: "async function() {return 4}",
	want: 4,
	},
	}
	for _, tt := range tests {
	tt := tt
	t.Run(tt.name, func(t *testing.T) {
	t.Parallel()
	vu, _, _, cleanUp := startIteration(t)
	defer cleanUp()
	// Test script as non string input
	vu.SetVar(t, "p", &sobek.Object{})
	got := vu.RunPromise(t, `
	p = await browser.newPage()
	return await p.evaluate(%s)
	`, tt.script)
	assert.Equal(t, vu.ToSobekValue(tt.want), got.Result())
	// Test script as string input
	got = vu.RunPromise(t,
	`return await p.evaluate("%s")`,
	tt.script,
	)
	assert.Equal(t, vu.ToSobekValue(tt.want), got.Result())
	})
	}
	}

Originally posted by @inancgumus in #1421 (comment)