Deny warnings in rustdoc non-UI tests
These warnings were silently ignored since they did not appear in a
`.stderr` file and did not fail the test. With this change, warnings in
tests are denied, causing the tests to fail if they have warnings.
This change has already led me to find a bug in rustdoc (#91274) and a
useless test (`src/test/rustdoc/primitive/primitive-generic-impl.rs`,
though its uselessness is unrelated to its warnings).
r? `@jyn514`
3c51718ae7