tests: add Trait.requiresXcode

.github/workflows/integration.yml

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+on:
+  push:
+    branches:
+      - main
+  pull_request:
+concurrency:
+  group: integration-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+jobs:
+  integration-test-linux:
+    runs-on: ubuntu-on-macos
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+      - name: Build and test
+        run: |
+          docker compose run --build -e XTL_CI=1 \
+            -v /usr/local/share/Xcode.app:/usr/local/share/Xcode.app xtool \
+            swift test

Tests/XToolTests/RequiresXcodeTrait.swift

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+import Foundation
+import Testing
+import Subprocess
+
+enum TestXcode {
+    /// A path to a copy of Xcode.app.
+    /// 
+    /// Tests that use this must be annotated with `Trait.requiresXcode`.
+    static func current(sourceLocation: SourceLocation = #_sourceLocation) -> URL {
+        guard let url = TestXcode._current else {
+            Issue.record(
+                "TestXcode.current was used without Trait.requiresXcode()",
+                sourceLocation: sourceLocation
+            )
+            return URL(filePath: "/dev/null")
+        }
+        return url
+    }
+
+    @TaskLocal fileprivate static var _current: URL?
+
+    fileprivate static let resolved = Task<URL?, Error> {
+        if let xcode = ProcessInfo.processInfo.environment["XTL_TEST_XCODE_APP"], !xcode.isEmpty {
+            return URL(filePath: xcode)
+        }
+        #if os(macOS)
+        if
+            let output = try await Subprocess.run(
+                .path("/usr/bin/xcode-select"),
+                arguments: ["-p"],
+                output: .string(limit: .max, encoding: UTF8.self),
+            ).standardOutput?.trimmingCharacters(in: .whitespacesAndNewlines),
+            output.hasSuffix(".app/Contents/Developer") {
+            return URL(filePath: output).deletingLastPathComponent().deletingLastPathComponent()
+        }
+        #endif
+        let candidate = URL(filePath: "/usr/local/share/Xcode.app")
+        if FileManager.default.fileExists(atPath: candidate.path) {
+            return candidate
+        }
+        return nil
+    }
+}
+
+struct RequiresXcodeTrait: TestTrait, SuiteTrait, TestScoping {
+    func provideScope(
+        for test: Test,
+        testCase: Test.Case?,
+        performing function: () async throws -> Void
+    ) async throws {
+        guard let xcode = try? await TestXcode.resolved.value else {
+            try Test.cancel("Could not resolve Xcode.app path. Set XTL_TEST_XCODE_APP.")
+        }
+        return try await TestXcode.$_current.withValue(xcode) {
+            try await function()
+        }
+    }
+}
+
+extension Trait where Self == RequiresXcodeTrait {
+    static var requiresXcode: Self {
+        Self()
+    }
+}

Tests/XToolTests/XcodeTests.swift

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+import Testing
+import Foundation
+
+@Test(.requiresXcode) func validateXcode() {
+    let xcode = TestXcode.current()
+    let swift = xcode.appending(path: "Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift")
+    #expect(
+        FileManager.default.fileExists(atPath: swift.path),
+        "Invalid or unrecognized layout for Xcode.app at \(xcode.path)"
+    )
+}