Rollup merge of #122355 - fmease:rustdoc-fix-up-old-test, r=notriddle

rustdoc: fix up old test

`tests/rustdoc/line-breaks.rs` had several issues:

1. It used `//`@count`` instead of `// `@count`` (notice the space!) which gets treated as a `ui_test` directive instead of a `htmldocck` one. `compiletest` didn't flag it as an error because it's allowlisted ([#121561](https://github.com/rust-lang/rust/pull/121561)) presumably precisely because of this test. And before the compiletest→ui_test migration, these directives must've been ignored, too, because …
2. … the checks themselves no longer work either: The count of `<br>`s is actually 0 in all 3 cases because – well – we no longer generate any `<br>`s inside `<pre>`s.

Since I don't know how to ``@count`` `\n`s instead of `<br>`s, I've turned them into ``@matches`.` Btw, I don't know if this test is still desirable or if we have other tests that cover this (I haven't checked).

r? rustdoc

src/tools/compiletest/src/header.rs

@@ -694,7 +694,6 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "check-stdout",
     "check-test-line-numbers-match",
     "compile-flags",
-    "count",
     "dont-check-compiler-stderr",
     "dont-check-compiler-stdout",
     "dont-check-failure-status",