Add test to ensure that the docblock elements left margin is as expected

Author: GuillaumeGomez

src/test/rustdoc-gui/check-code-blocks-margin.goml

@@ -0,0 +1,6 @@
+// This test ensures that the docblock elements have the appropriate left margin.
+goto: file://|DOC_PATH|/test_docs/fn.foo.html
+// The top docblock elements shouldn't have left margin...
+assert-css: ("#main .docblock.item-decl", {"margin-left": "0px"})
+// ... but all the others should!
+assert-css: ("#main .docblock:not(.item-decl)", {"margin-left": "24px"})