Reland "Build iOS unittest target in unopt builds" (#44356)" (#44821)

Relands #44301

The original PR was reverted in #44356

The `assert flutter_dylib_time <= ios_test_lib_time, final_message` check is reverted in 70dd9e4, so we are safe to land this again. 

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: Chris Yang

BUILD.gn

@@ -149,6 +149,10 @@ group("unittests") {
         [ "//flutter/shell/platform/android:flutter_shell_native_unittests" ]
   }
 
+  if (is_ios) {
+    public_deps += [ "//flutter/shell/platform/darwin/ios:ios_test_flutter" ]
+  }
+
   # Compile all unittests targets if enabled.
   if (enable_unittests) {
     public_deps += [

shell/platform/darwin/ios/BUILD.gn

@@ -270,7 +270,7 @@ source_set("ios_test_flutter_mrc") {
 
 shared_library("ios_test_flutter") {
   testonly = true
-  visibility = [ ":*" ]
+  visibility = [ "*" ]
   cflags = [
     "-fvisibility=default",
     "-F$platform_frameworks_path",