message: add build script failure test when requesting backtraces
  it displays an additional message on how to improve these backtraces, now that
  debuginfo is turned off most of the time in `dev.build-override`.
diff: @@ -47,6 +47,44 @@ Caused by:
  .run();
  }
  +#[cargo_test]
  +fn custom_build_script_failed_backtraces_message() {
  + // In this situation (no dependency sharing), debuginfo is turned off in
  + // `dev.build-override`. However, if an error occurs running e.g. a build
  + // script, and backtraces are opted into: a message explaining how to
  + // improve backtraces is also displayed.
  + let p = project()
  + .file(
  + "Cargo.toml",
  + r#"
  + [package]
  +
  + name = "foo"
  + version = "0.5.0"
  + authors = ["wycats@example.com"]
  + build = "build.rs"
  + "#,
  + )
  + .file("src/main.rs", "fn main() {}")
  + .file("build.rs", "fn main() { std::process::exit(101); }")
  + .build();
  + p.cargo("build -v")
  + .env("RUST_BACKTRACE", "1")
  + .with_status(101)
  + .with_stderr(
  + "\
  +[COMPILING] foo v0.5.0 ([CWD])
  +[RUNNING] `rustc --crate-name build_script_build build.rs [..]--crate-type bin [..]`
  +[RUNNING] `[..]/build-script-build`
  +[ERROR] failed to run custom build command for `foo v0.5.0 ([CWD])`
  +note: To improve backtraces for build dependencies[..]
  +
  +Caused by:
  + process didn't exit successfully: `[..]/build-script-build` (exit [..]: 101)",
  + )
  + .run();
  +}
  +
  #[cargo_test]
  fn custom_build_env_vars() {
  let p = project()
  