test for issue 3164

[ytmimi]

Closes #3164

Show that when error_on_line_overflow=true is set in the rustfmt.toml that an error message is written to stderr.

[ytmimi]

The "stable" tests were failing because we couldn't set error_on_line_overflow=true via a rustfmt.toml. I've removed the rustfmt.toml that I was using for testing, and I've updated each test to pass the configuration value via the command line with --config=error_on_line_overflow=true.

[calebcartwright]

Why the cargo-fmt test? Seems duplicative of the rustfmt test at first blush

[ytmimi]

The original issue mentioned cargo-fmt, so I figured for completeness I'd add a test where I invoke cargo-fmt

[calebcartwright]

The original issue mentioned cargo-fmt, so I figured for completeness I'd add a test where I invoke cargo-fmt

Gotcha, in general unless there's something specific to cargo fmt (as opposed to that just being the user's entry point) there's no need to test something at the rustfmt level through cargo fmt too. However, it's not much extra code so fine here