There are some tests in the rustfmt test suite that are ignored by
default. I believe these tests are ignored because they have caused
issues with the the `rust-lang/rust` test suite.
However, we recently experienced an issue (5395) that would have been
avoided had these tests been running.
With the introduction of the new `#[rustfmt_only_ci_test]` attribute
macro we can run these tests when the `RUSTFMT_CI` environment variable
is set, which will presumably only be set during rustfmts CI runs.
When the environment variable is not set the `#[rustfmt_only_ci_test]`
will be replaced with an `#[ignore]`.
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