Rename ExitTestArtifacts and split ExitCondition in twain. (#975)

This PR renames `ExitTestArtifacts` to `ExitTest.Result` and splits
`ExitCondition` into two types: `ExitTest.Condition` which can be passed
to `#expect(exitsWith:)` and `StatusAtExit` which represents the raw,
possibly platform-specific status reported by the kernel when a child
process terminates.

The latter type is not nested in `ExitTest` because it can be used
independently of exit tests and we may want to use it in the future for
things like multi-process parallelization, but if a platform supports
spawning processes but not exit tests, nesting it in `ExitTest` would
make it unavailable.

I considered several names for `StatusAtExit`:

- `ExitStatus`: too easily confusable with exit _codes_ such as
`EXIT_SUCCESS`;
- `ProcessStatus`: we don't say "process" in our API surface elsewhere;
- `Status`: too generic
- `ExitReason`: "status" is a more widely-used term of art for this
concept.

Foundation uses `terminationStatus` to represent the raw integer value
and `Process.TerminationReason` to represent whether it's an exit code
or signal. We don't use "termination" in Swift Testing's API anywhere.

I settled on `StatusAtExit` because it was distinct and makes it clear
that it represents the status of a process _at exit time_ (as opposed to
while running, e.g. `enum ProcessStatus { case running; case suspended;
case terminated }`.

Updates to the exit tests proposal document will follow in a separate
PR.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/CMakeLists.txt

@@ -31,10 +31,11 @@ add_library(Testing
   Events/Recorder/Event.JUnitXMLRecorder.swift
   Events/Recorder/Event.Symbol.swift
   Events/TimeValue.swift
-  ExitTests/ExitCondition.swift
   ExitTests/ExitTest.swift
-  ExitTests/ExitTestArtifacts.swift
+  ExitTests/ExitTest.Condition.swift
+  ExitTests/ExitTest.Result.swift
   ExitTests/SpawnProcess.swift
+  ExitTests/StatusAtExit.swift
   ExitTests/WaitFor.swift
   Expectations/Expectation.swift
   Expectations/Expectation+Macro.swift