Port {{concat}} helper test to ember-glimmer

[chancancode]

This is a 1:1 port of https://github.com/emberjs/ember.js/blob/master/packages/ember-htmlbars/tests/helpers/concat-test.js to the ember-glimmer test directory, with some minor cleanup. We don't have {{#if}} working yet (working on that next), so I split the "can be used as a sub-expression" test into two (which imo, seemed like a good refactor anyway).

The test is sym-linked back into ember-htmlbars as usual, so it is ran in both packages.

Given that this is 1:1 with the existing test, and it is ran in both packages, is it okay if we delete the existing version here? (I would like to do that, so we can easily keep track of what we have made ported.)

cc @rwjblue

[rwjblue]

This seems to duplicate a few things:

• packages/ember-glimmer/lib/helpers/-concat.js looks like a copy of packages/ember-htmlbars/lib/helpers/-concat.js
• packages/ember-glimmer/lib/helper.js likes like a copy of packages/ember-htmlbars/lib/helper.js

Can these be symlinks instead?

[rwjblue]

Given that this is 1:1 with the existing test, and it is ran in both packages, is it okay if we delete the existing version here?

Yes, I think deleting is totally fine as we port the tests. The key thing to me is that the same tests have to be run in both contexts, and the new tests have to faithfully represent the intent of the existing tests (which I think you have done well here).

[chancancode]

I symlinked the -concat.js (and renamed it to concat.js). I kept helper.js as is because they have some small differences (and will likely diverge more).