introduce build boilerplate for ios unit tests (#37811)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/37811

Changelog: [Internal]

i am looking to add ios unit tests to venice and this is the first unit test suite that will test native ios code in the new architecture afaik, so i wanted to open this up to discussion.

currently, all `XCTest` in `react-native-github` are coupled with the `RNTester` target. my main qualm with this is i am concerned that it won't scale well. currently we have only ~30ish tests but ultimately if we want a proper testing suite, surely this count will be in the hundreds and that won't be able to reasonably live in a single test target.

however, the trade is that this test will not show up in RNTester. i have added a unit test to RNTester before in D31949237, however the experience was extremely painful as i had to manually update the `project.pbxproj` to include my file, and i had to manually determine what hex value was the next one (for whatever reason, this doesn't increment at the endian...).

i am wondering if we can treat the current unit testing experience in RNTester as pretty much maintenance mode and start thinking of a improved version starting with something more modular like this.

Reviewed By: cipolleschi

Differential Revision: D46467229

fbshipit-source-id: 09de9cf8bc5f8b9c86abcaf7750a6f63686d8d1a

packages/react-native/ReactCommon/react/bridgeless/iostests/RCTHostTests.mm

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import <XCTest/XCTest.h>
+
+@interface RCTHostTests : XCTestCase
+@end
+
+@implementation RCTHostTests
+
+- (void)testHost
+{
+}
+
+@end