rustdoc: fix up old test

[fmease]

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) 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 \ns 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