Implemented the TodoGroup enumeration in the app target.

UnitTests/Tests/Enumerations/TodoGroupTests.swift

@@ -0,0 +1,99 @@
+import Foundation
+import SwiftUI
+import Testing
+
+@testable import Todos
+
+@Suite("TodoGroup enumeration tests")
+struct TodoGroupTests {
+
+    // MARK: Computed tests
+
+    @Test(
+        "group color match expected values",
+        arguments: zip(
+            TodoGroup.allCases,
+            [Color.blue, .green, .red, .gray]
+        )
+    )
+    func color(
+        for group: TodoGroup,
+        expects expectation: Color
+    ) {
+        // GIVEN
+        // WHEN
+        let color = group.color
+
+        // THEN
+        #expect(color == expectation)
+    }
+
+    @Test(
+        "group icon names match expected values",
+        arguments: zip(
+            TodoGroup.allCases,
+            ["calendar.badge.clock", "calendar", "calendar.badge.exclamationmark", "checkmark.circle.fill"]
+        )
+    )
+    func iconName(
+        for group: TodoGroup,
+        expects expectation: String
+    ) {
+        // GIVEN
+        // WHEN
+        let iconName = group.iconName
+
+        // THEN
+        #expect(iconName == expectation)
+    }
+
+    @Test(
+        "group titles match expected values",
+        arguments: zip(
+            TodoGroup.allCases,
+            ["today", "upcoming", "overdue", "completed"]
+        )
+    )
+    func title(
+        for group: TodoGroup,
+        expects expectation: String
+    ) {
+        // GIVEN
+        let key = "enumeration.todo-group.title." + expectation
+
+        // WHEN
+        let title = group.title
+
+        // THEN
+        #expect(title == .init(key))
+    }
+
+    // MARK: Case Iteration Tests
+
+    @Test("all cases contains all cases, in correct order")
+    func allCases() {
+        // GIVEN
+        // WHEN
+        let allCases = TodoGroup.allCases
+
+        // THEN
+        #expect(allCases.count == 4)
+        #expect(allCases[0] == .today)
+        #expect(allCases[1] == .upcoming)
+        #expect(allCases[2] == .overdue)
+        #expect(allCases[3] == .completed)
+    }
+
+    // MARK: Identifiable Tests
+
+    @Test("id returns an identity of a case", arguments: TodoGroup.allCases)
+    func id(for group: TodoGroup) {
+        // GIVEN
+        // WHEN
+        let id = group.id
+
+        // THEN
+        #expect(id == group)
+    }
+
+}