Fix main actor isolation warnings that are Swift 6 errors in the XCUITests suite.

Author: ih-codes

firefox-ios/firefox-ios-tests/Tests/XCUITests/A11yHomePageTests.swift

@@ -4,6 +4,7 @@
 
 import XCTest
 
+@MainActor
 class A11yHomePageTests: BaseTestCase {
     func testA11yHomePageAudit() throws {
         guard #available(iOS 17.0, *), !skipPlatform else { return }

firefox-ios/firefox-ios-tests/Tests/XCUITests/A11ySearchTest.swift

@@ -5,6 +5,7 @@
 import XCTest
 import Common
 
+@MainActor
 class A11ySearchTest: BaseTestCase {
     private func typeTextAndValidateSearchSuggestions(text: String, isSwitchOn: Bool) {
         typeOnSearchBar(text: text)

firefox-ios/firefox-ios-tests/Tests/XCUITests/A11ySettingsTests.swift

@@ -4,6 +4,7 @@
 
 import XCTest
 
+@MainActor
 class A11ySettingsTests: BaseTestCase {
     func testSettingsMenuPageAudit() throws {
         guard #available(iOS 17.0, *), !skipPlatform else { return }