Allow enabling traits for test targets in the package manifest

[ptoffy]

Description

If a trait is disabled by default, there’s currently no way to enable it for testing within its package. It would be useful to be able to enable traits on a target from the manifest; otherwise, the only way to use them is by running swift test --enable-all-traits or swift test --traits Foo, which can be a bit awkward.

Expected behavior

Be able to do something like

.testTarget(
    name: "PackageTests",
    dependencies: [
        "Package"
    ],
    swiftSettings: [.enable(traits: ["Trait"])]
),

Actual behavior

Tests using a trait, hidden by a #if Trait condition, are not run.
We can use swiftSettings: [.define("Trait")] however that enables the trait only in the test target and leads to compile errors because code in the sources (used by the test target) is still hidden by the #if condition and is therefore not compiled.

Steps to reproduce

No response

Swift Package Manager version/commit hash

No response

Swift & OS version (output of swift --version && uname -a)

Apple Swift version 6.1 (swift-6.1-RELEASE)
Target: arm64-apple-macosx16.0
Darwin Pauls-MacBook-Air.local 25.1.0 Darwin Kernel Version 25.1.0: Mon Oct 20 19:34:03 PDT 2025; root:xnu-12377.41.6~2/RELEASE_ARM64_T8112 arm64