[CMake] Fix build after 313638@main by using canonical include path for IOSMouseEventTestHarness.h

https://bugs.webkit.org/show_bug.cgi?id=315260
rdar://177590639

Reviewed by Charlie Wolfe and BJ Burg.

SwitchInputTests.mm imported "IOSMouseEventTestHarness.h" with a bare
filename. This resolved under the Xcode build because of its header
search paths, but broke the Mac CMake build, which only sees the header
via its repo-relative path.

Match the convention used by every other consumer of this header
(SampledPageTopColor.mm, UIDelegate.mm, UnifiedPDFTests.mm,
iOSMouseSupport.mm, NavigationAction.mm) and the harness's own .mm:
import as "Helpers/ios/IOSMouseEventTestHarness.h".

* Tools/TestWebKitAPI/Tests/WebKit/WKWebView/SwitchInputTests.mm:

Canonical link: https://commits.webkit.org/313645@main

Author: zakariaridouh

Tools/TestWebKitAPI/Tests/WebKit/WKWebView/SwitchInputTests.mm

@@ -27,7 +27,7 @@
 
 #if PLATFORM(COCOA)
 
-#import "IOSMouseEventTestHarness.h"
+#import "Helpers/ios/IOSMouseEventTestHarness.h"
 #import "InstanceMethodSwizzler.h"
 #import "Helpers/PlatformUtilities.h"
 #import "Helpers/Test.h"