Make --backend:cpp|js work for :test: code-blocks as well (#14306)

Continues 9502e39b63

Ref:
- 9502e39b63 (commitcomment-39087584)
- https://github.com/nim-lang/Nim/pull/14278

Fixes https://github.com/nim-lang/Nim/issues/13129 .

lib/packages/docutils/rstgen.nim

@@ -865,7 +865,7 @@ proc parseCodeBlockField(d: PDoc, n: PRstNode, params: var CodeBlockParams) =
   of "test":
     params.testCmd = n.getFieldValue.strip
     if params.testCmd.len == 0:
-      params.testCmd = "nim c -r $1"
+      params.testCmd = "nim <backend> -r $1" # The nim backend is auto-set in docgen.nim
     else:
       params.testCmd = unescape(params.testCmd)
   of "status", "exitcode":