[glimmer2] fix unbound helper test

[fivetanley]

Before, the test rendered the following:

<div data-foo="{{if cond1 "T1" "F1"}}"></div>
<div data-foo="{{if cond2 "T2" "F2"}}"></div>

The test was only asserting against the value of the data-foo of the
first div, causing it fo fail because it was missing the value of the
data-foo in the second div.

This is because the test was using wrappedTemplateFor, which
is a convenience method which only expects one template to be
rendered at a time.

Instead, we use wrapperFor and two templateFor calls to instead
create only one div and assert against it. What's rendered in the test
now looks like:

<div data-foo="{{if cond1 "T1" "F1"}}{{if cond2 "T2" "F2"}}"></div>

refs #13644

[rwjblue]

Thanks @fivetanley!