message: Add some contributor docs for debugging testsuite tests.
diff: @@ -123,6 +123,34 @@ If you need to test with registry dependencies, see
  If you need to test git dependencies, see [`support::git`] to create a git
  dependency.
  +## Debugging tests
  +
  +In some cases, you may need to dig into a test that is not working as you
  +expect, or you just generally want to experiment within the sandbox
  +environment. The general process is:
  +
  +1. Build the sandbox for the test you want to investigate. For example:
  +
  + `cargo test --test testsuite -- features2::inactivate_targets`.
  +2. In another terminal, head into the sandbox directory to inspect the files and run `cargo` directly.
  + 1. The sandbox directories start with `t0` for the first test.
  +
  + `cd target/tmp/cit/t0`
  + 2. Set up the environment so that the sandbox configuration takes effect:
  +
  + `export CARGO_HOME=$(pwd)/home/.cargo`
  + 3. Most tests create a `foo` project, so head into that:
  +
  + `cd foo`
  +3. Run whatever cargo command you want. See [Running Cargo] for more details
  + on running the correct `cargo` process. Some examples:
  +
  + * `/path/to/my/cargo/target/debug/cargo check`
  + * Using a debugger like `lldb` or `gdb`:
  + 1. `lldb /path/to/my/cargo/target/debug/cargo`
  + 2. Set a breakpoint, for example: `b generate_targets`
  + 3. Run with arguments: `r check`
  +
  [`testsuite`]: https://github.com/rust-lang/cargo/tree/master/tests/testsuite/
  [`ProjectBuilder`]: https://github.com/rust-lang/cargo/blob/e4b65bdc80f2a293447f2f6a808fa7c84bf9a357/crates/cargo-test-support/src/lib.rs#L225-L231
  [`Execs`]: https://github.com/rust-lang/cargo/blob/e4b65bdc80f2a293447f2f6a808fa7c84bf9a357/crates/cargo-test-support/src/lib.rs#L558-L579
  @@ -130,3 +158,4 @@ dependency.
  [`support::compare`]: https://github.com/rust-lang/cargo/blob/master/crates/cargo-test-support/src/compare.rs
  [`support::registry::Package`]: https://github.com/rust-lang/cargo/blob/e4b65bdc80f2a293447f2f6a808fa7c84bf9a357/crates/cargo-test-support/src/registry.rs#L73-L149
  [`support::git`]: https://github.com/rust-lang/cargo/blob/master/crates/cargo-test-support/src/git.rs
  +[Running Cargo]: ../process/working-on-cargo.md#running-cargo
  