Why can I not break the test suite

I can run the test suite as described in CONTRIBUTING.md with mage -v. Everything passes.

I then tried to run a single test:

$ go test -run ./hugolib/embedded_shortcodes_test.go
testing: warning: no tests to run
PASS
ok  	github.com/gohugoio/hugo	0.049s

Huh, OK, so I assume the tests are being bundled into some other asset that gets run. Let’s just run the whole mage thing again and find out where the test gets run if we assuredly break the test suite with these changes:

$ git diff|cat
diff --git a/hugolib/embedded_shortcodes_test.go b/hugolib/embedded_shortcodes_test.go
index 5188be2c9..b5fb693c7 100644
--- a/hugolib/embedded_shortcodes_test.go
+++ b/hugolib/embedded_shortcodes_test.go
@@ -83,7 +83,7 @@ Content: {{ .Content }}|
 https://gist.github.com/spf13/7896402.js
 <span style="color:#a6e22e">main</span></span>
 https://t.co/X94FmYDEZJ
-https://www.youtube.com/embed/PArFPgHrNZM
+https://www.youtube.com/embed/FUNKY_FOO
 Foo: bar
 
 
diff --git a/tpl/tplimpl/embedded/templates/shortcodes/youtube.html b/tpl/tplimpl/embedded/templates/shortcodes/youtube.html
index 93fed2326..032ed82fb 100644
--- a/tpl/tplimpl/embedded/templates/shortcodes/youtube.html
+++ b/tpl/tplimpl/embedded/templates/shortcodes/youtube.html
@@ -5,6 +5,6 @@
 {{- $class := .Get "class" | default (.Get 1) -}}
 {{- $title := .Get "title" | default "YouTube Video" }}
 <div {{ with $class }}class="{{ . }}"{{ else }}style="position: relative; padding-bottom: 56.25%; height: 0; overflow: hidden;"{{ end }}>
-  <iframe src="https://{{ $ytHost }}/embed/{{ $id }}{{ with .Get "autoplay" }}{{ if eq . "true" }}?autoplay=1{{ end }}{{ end }}" {{ if not $class }}style="position: absolute; top: 0; left: 0; width: 100%; height: 100%; border:0;" {{ end }}allowfullscreen title="{{ $title }}"></iframe>
+  <iframe src="https://{{ $ytHost }}/{{ with .Get "autoplay" }}{{ if eq . "true" }}?autoplay=1{{ end }}{{ end }}" {{ if not $class }}style="position: absolute; top: 0; left: 0; width: 100%; height: 100%; border:0;" {{ end }}allowfullscreen title="{{ $title }}"></iframe>
 </div>
 {{ end -}}

But oddly, the suite passes.

$ echo $?
0

Are there no meaningful tests for the {{< youtube >}} embed? Or am I Running the Tests Wrong™. I’m sure it’s the latter, but I don’t know how to navigate this test toolchain. I just want to land a minor tweak to the shortcode, but I want to ensure that any changes come with a validating test.

I feel like this may have slipped through the cracks so I’m replying for visibility.

Short version is that I’m still trying to run one-off tests to prove that I’m not introducing a regression. I don’t see my case covered in CONRIBUTING.md and I don’t know how to get started helping without the ability to validate my work. Thanks in advance.