Skip testWasm if the host toolchain can’t build for WASM

This fixes a test failure when running tests using Xcode 15.4.

Author: ahoppen

Sources/SKTestSupport/SkipUnless.swift

@@ -462,6 +462,24 @@ public actor SkipUnless {
       }
     }
   }
+
+  public static func canCompileForWasm(
+    file: StaticString = #filePath,
+    line: UInt = #line
+  ) async throws {
+    return try await shared.skipUnlessSupportedByToolchain(swiftVersion: SwiftVersion(6, 0), file: file, line: line) {
+      let swiftFrontend = try await unwrap(ToolchainRegistry.forTesting.default?.swift).parentDirectory
+        .appending(component: "swift-frontend")
+      return try await withTestScratchDir { scratchDirectory in
+        let input = scratchDirectory.appending(component: "Input.swift")
+        FileManager.default.createFile(atPath: input.pathString, contents: nil)
+        let process = Process(args: swiftFrontend.pathString, "--version")
+        try process.launch()
+        let result = try await process.waitUntilExit()
+        return result.exitStatus == .terminated(code: 0)
+      }
+    }
+  }
 }
 
 // MARK: - Parsing Swift compiler version

Tests/SourceKitLSPTests/SwiftPMIntegration.swift

@@ -243,6 +243,8 @@ final class SwiftPMIntegrationTests: XCTestCase {
   }
 
   func testWasm() async throws {
+    try await SkipUnless.canCompileForWasm()
+
     let project = try await SwiftPMTestProject(
       files: [
         "/.sourcekit-lsp/config.json": """