Compiletest Directives: Environment, Output Forbidding, Bug Handling, and Test Execution Control

4th chunk of `src/tests/directives.md`

6a21a5ba105b4a16f4a2e6f7e207b06fcdfb29ba164fa9930000000100000fb2

| `dont-check-compiler-stdout`      | Don't check actual compiler stdout vs stdout snapshot                                                                    | `ui`                                         | N/A                                                                                     |
| `dont-require-annotations`        | Don't require line annotations for the given diagnostic kind (`//~ KIND`) to be exhaustive                               | `ui`, `incremental`                          | `ERROR`, `WARN`, `NOTE`, `HELP`, `SUGGESTION`                                           |
| `run-rustfix`                     | Apply all suggestions via `rustfix`, snapshot fixed output, and check fixed output builds                                | `ui`                                         | N/A                                                                                     |
| `rustfix-only-machine-applicable` | `run-rustfix` but only machine-applicable suggestions                                                                    | `ui`                                         | N/A                                                                                     |
| `exec-env`                        | Env var to set when executing a test                                                                                     | `ui`, `crashes`                              | `<KEY>=<VALUE>`                                                                         |
| `unset-exec-env`                  | Env var to unset when executing a test                                                                                   | `ui`, `crashes`                              | Any env var name                                                                        |
| `stderr-per-bitwidth`             | Generate a stderr snapshot for each bitwidth                                                                             | `ui`                                         | N/A                                                                                     |
| `forbid-output`                   | A pattern which must not appear in stderr/`cfail` output                                                                 | `ui`, `incremental`                          | Regex pattern                                                                           |
| `run-flags`                       | Flags passed to the test executable                                                                                      | `ui`                                         | Arbitrary flags                                                                         |
| `known-bug`                       | No error annotation needed due to known bug                                                                              | `ui`, `crashes`, `incremental`               | Issue number `#123456`                                                                  |

    where the test binary's stdout and stderr gets concatenated and then
    `error-pattern`s are matched on this combined output, which is ??? slightly
    questionable to say the least.

### Controlling when tests are run

These directives are used to ignore the test in some situations, which
means the test won't be compiled or run.

* `ignore-X` where `X` is a target detail or other criteria on which to ignore the test (see below)
* `only-X` is like `ignore-X`, but will *only* run the test on that target or
  stage
* `ignore-auxiliary` is intended for files that *participate* in one or more other
  main test files but that `compiletest` should not try to build the file itself.
  Please backlink to which main test is actually using the auxiliary file.
* `ignore-test` always ignores the test. This can be used to temporarily disable
  a test if it is currently not working, but you want to keep it in tree to
  re-enable it later.

Some examples of `X` in `ignore-X` or `only-X`:

- A full target triple: `aarch64-apple-ios`

Title: Compiletest Directives: Environment, Output Forbidding, Bug Handling, and Test Execution Control

Summary

This section outlines compiletest directives for manipulating the test execution environment (`exec-env`, `unset-exec-env`), managing bitwidth-specific output (`stderr-per-bitwidth`), forbidding patterns in output (`forbid-output`), passing flags to the test executable (`run-flags`), and acknowledging known bugs (`known-bug`). It also covers directives for controlling when tests are run, including `ignore-X`, `only-X`, `ignore-auxiliary`, and `ignore-test`, which allow selective disabling or enabling of tests based on target details or other criteria.