tests for only dark brand/theme

these implicitly create a light theme as well
and will include two stylesheets

ref #12400

Author: gordonwoodhull

tests/docs/smoke-all/dark-mode/dark-brand.qmd

@@ -0,0 +1,17 @@
+---
+format: html
+brand:
+  dark: slate-brand.yml
+_quarto:
+  tests:
+    html:
+      ensureHtmlElements:
+        - ["link[href*=\"-dark-\"]"]
+        - []
+---
+
+## Hello
+
+```markdown
+![asdf](asd.png)
+```

tests/docs/smoke-all/dark-mode/dark-theme.qmd

@@ -0,0 +1,17 @@
+---
+format: revealjs
+theme:
+  dark: slate
+_quarto:
+  tests:
+    html:
+      ensureHtmlElements:
+        - ["link[href*=\"-dark-\"]"]
+        - []
+---
+
+## Hello
+
+```markdown
+![asdf](asd.png)
+```