Rollup merge of #137099 - yotamofek:pr/rustdoc/fix-ignored-test-directives, r=fmease

Fix rustdoc test directives that were accidentally ignored 🧐

Replace "// `@"` with "//@ ", and fix the tests so they actually pass, after directives are checked.
~~Only the first commit is mandatory, other two are small drive-bys.~~

Author: tgross35