[ST-0010] Add ConditionTrait.evaluate()

proposals/testing/NNNN-evaluate-condition.md

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+# Public API to evaluate ConditionTrait
+
+* Proposal: [SWT-NNNN](NNNN-evaluate-condition.md)
+* Authors: [David Catmull](https://github.com/Uncommon)
+* Review Manager: [Stuart Montgomery](https://github.com/stmontgomery)
+* Status: **Awaiting review**
+* Bug: [swiftlang/swift-testing#903](https://github.com/swiftlang/swift-testing/issues/903)
+* Implementation: [swiftlang/swift-testing#909](https://github.com/swiftlang/swift-testing/pull/909)
+* Review: ([pitch](https://forums.swift.org/t/pitch-introduce-conditiontrait-evaluate/77242))
+
+## Introduction
+
+This adds an `evaluate()` method to `ConditionTrait` to evaluate the condition
+without requiring a `Test` instance.
+
+## Motivation
+
+Currently, the only way a `ConditionTrait` is evaluated is inside the
+`prepare(for:)` method. This makes it difficult for third-party libraries to
+utilize these traits because evaluating a condition would require creating a
+dummy `Test` to pass to that method.
+
+## Proposed solution
+
+The proposal is to add a `ConditionTrait.evaluate()` method which returns the
+result of the evaluation. The existing `prepare(for:)` method is updated to call
+`evaluate()` so that the logic is not duplicated.
+
+## Detailed design
+
+The `evaluate()` method is as follows, containing essentially the same logic
+as was in `prepare(for:)`:
+
+```swift
+extension ConditionTrait {
+  /// Evaluate this instance's underlying condition.
+  ///
+  /// - Returns: The result of evaluating this instance's underlying condition.
+  ///
+  /// The evaluation is performed each time this function is called, and is not
+  /// cached.
+  public func evaluate() async throws -> Bool
+}
+```
+
+## Source compatibility
+
+This change is purely additive.
+
+## Integration with supporting tools
+
+This change allows third-party libraries to apply condition traits at other
+levels than suites or whole test functions, for example if tests are broken up
+into smaller sections.
+
+## Future directions
+
+This change seems sufficient for third party libraries to make use of
+`ConditionTrait`. Changes for other traits can be tackled in separate proposals.
+
+## Alternatives considered
+
+Exposing `ConditionTrait.Kind` and `.kind` was also considered, but it seemed
+unnecessary to go that far, and it would encourage duplicating the logic that
+already exists in `prepare(for:)`.