tpl/tplimpl: Fix context in shortcode error messages

[jmooring]

Fixes #13279

OK, so this PR does a bit more than advertised, primarily moving the shortcode integration tests into their own file, and adding a few more.

For shortcodes that render more than a few lines, these tests compare content hashes. It may take a little more time to figure out why one of these tests fails in the future, but this does a much better job of detecting the failure.

[bep]

So, I like the concept of using hashes here, but I suspect I'm going to get bitten by the lack of output at some point.

I suggest that we print both the hash and the content, e.g.

Hash: {{ .Content | hash.XxHash }}
Content: {{ .Content }}

And then do:

b.AssertFileContent("public/index.html", "a7937c49665872d3")

The test will be as robust as before, but we will get better output on failures.