Implemented the "exists(at: )" function for the FileService service in the module target.

Tests/Library/Services/FileServiceTests.swift

@@ -1,22 +1,69 @@
+import Foundation
 import Testing
 
 @testable import ColibriLibrary
 
 struct FileServiceTests {
+    
+    // MARK: Properties
+    
+    private let service: FileServicing
+    
+    // MARK: Initialisers
+    
+    init() {
+        self.service = FileService()
+    }
 
     // MARK: Properties tests
 
-    @Test("Test the file service provides a current folder URL")
-    func currentFolder() async {
+    @Test func currentFolder() async {
         // GIVEN
-        let service = FileService()
-
         // WHEN
         let url = await service.currentFolder
 
         // THEN
-        #expect(url.path() == "/private/tmp")
+        #expect(url == .someExistingFolder)
         #expect(url.isFileURL == true)
     }
+        // GIVEN
+        // WHEN
+        // THEN
+    }
+    
+    @Test(arguments: zip([URL.someExistingFolder, .someExistingFile, .someNonExistingFolder, .someNonExistingFile],
+                         [true, true, false, false]))
+    func exists(
+        with url: URL,
+        expects outcome: Bool
+    ) async throws {
+        // GIVEN
+        // WHEN
+        let result = try await service.exists(at: url)
+        
+        // THEN
+        #expect(result == outcome)
+    }
+    
+    @Test func existsThrows() async throws {
+        // GIVEN
+        let url = URL.someRandomURL
+
+        // WHEN
+        // THEN
+        await #expect(throws: FileServiceError.urlNotFileURL) {
+            try await service.exists(at: url)
+        }
+    }
 
 }
+
+// MARK: - URL+Constants
+
+private extension URL {
+    static let someExistingFolder = URL(at: "/private/tmp")
+    static let someExistingFile = URL(at: "/etc/null")
+    static let someNonExistingFolder = URL(at: "/some/random/folder")
+    static let someNonExistingFile = URL(at: "/some/random/file.ext")
+    static let someRandomURL = URL(string: "https://some.random.url")!
+}