Report whether a source file is part of a test target in the SourceKit-LSP API (#7978)

Companion of swiftlang/sourcekit-lsp#1685

Author: ahoppen

Sources/SourceKitLSPAPI/BuildDescription.swift

@@ -45,16 +45,20 @@ public protocol BuildTarget {
     /// Whether the target is part of the root package that the user opened or if it's part of a package dependency.
     var isPartOfRootPackage: Bool { get }
 
+    var isTestTarget: Bool { get }
+
     func compileArguments(for fileURL: URL) throws -> [String]
 }
 
 private struct WrappedClangTargetBuildDescription: BuildTarget {
     private let description: ClangModuleBuildDescription
     let isPartOfRootPackage: Bool
+    let isTestTarget: Bool
 
     init(description: ClangModuleBuildDescription, isPartOfRootPackage: Bool) {
         self.description = description
         self.isPartOfRootPackage = isPartOfRootPackage
+        self.isTestTarget = description.isTestTarget
     }
 
     public var sources: [URL] {
@@ -87,10 +91,12 @@ private struct WrappedClangTargetBuildDescription: BuildTarget {
 private struct WrappedSwiftTargetBuildDescription: BuildTarget {
     private let description: SwiftModuleBuildDescription
     let isPartOfRootPackage: Bool
+    let isTestTarget: Bool
 
     init(description: SwiftModuleBuildDescription, isPartOfRootPackage: Bool) {
         self.description = description
         self.isPartOfRootPackage = isPartOfRootPackage
+        self.isTestTarget = description.isTestTarget
     }
 
     public var name: String {

Sources/SourceKitLSPAPI/PluginTargetBuildDescription.swift

@@ -25,6 +25,7 @@ struct PluginTargetBuildDescription: BuildTarget {
     private let toolsVersion: ToolsVersion
     private let toolchain: any Toolchain
     let isPartOfRootPackage: Bool
+    var isTestTarget: Bool { false }
 
     init(target: ResolvedModule, toolsVersion: ToolsVersion, toolchain: any Toolchain, isPartOfRootPackage: Bool) {
         assert(target.type == .plugin)