Auto merge of #4592 - ehuss:check_test, r=alexcrichton

Auto merge of #4592 - ehuss:check_test, r=alexcrichton

Add unit test checking to `cargo check`

This is an extension of PR #4039, fixing #3431, #4003, #4059, #4330.  The fixes for #4059 can potentially be separated into a separate PR, though there may be some overlap.

The general gist of the changes:

- Add `--profile test` flag to `cargo check` that will enable checking of unit tests.
- `--tests` will implicitly enable checking of unit tests within the lib (checks the same targets as `cargo test`).  This affects the `check`, `test`, and `build` commands.
- `--benches` behaves similarly by using the same targets as `cargo bench`.
- Fix erroneously linking tests when run with `--test`.

There is one thing I did not do because I wanted more feedback on what others think the expected behavior should be.  What should the behavior of `--all-targets` be?  This patch does not (yet) make any changes to its behavior.  My initial thinking is that it should *add* a target of `--lib --bins --profile test`, but that essentially means the lib and bin targets will be checked twice (and thus any errors/warnings outside of `#[cfg(test)]` will appear twice, which may be confusing, and generally take twice as long to run).  I can add that, but I think it would require adding a new `All` variant to `CompileFilter` so that the code in `generate_targets` can detect this scenario.  I wanted feedback before making a more extensive change like that.  The downside of not adding it is that `--all-targets` will ignore unit tests (if you don't specify `--profile test`).

Summary of the profiles used with this patch:

Command                         | Lib               | Bin foo     | Test t1 | Example e1 | Bench b1 |
-------                         | ---               | -------     | ------- | ---------- | -------- |
`check`                         | check             | check       | -       | -          | -        |
`check --profile test`          | check_test†       | check_test† | -       | -          | -        |
`check --lib`                   | check             | -           | -       | -          | -        |
`check --lib --profile test`    | check_test†       | -           | -       | -          | -        |
`check --bin foo`               | check             | check       | -       | -          | -        |
`check -–bin foo –profile test` | check_test†       | check_test† | -       | -          | -        |
`check --bins`                  | check             | check       | -       | -          | -        |
`check --test t1`               | check             | check       | check_test   | -          | -        |
`check --tests`                 | check, check_test†  | check, check_test† | check_test | check†, check_test†  | -    |
`check --all-targets`           | check, check_test†  | check, check_test†  | check_test   | check, check_test† | check_test    |

† = different behavior from today