Embed OCMock and iOS tests into IosUnitTests app

[jmagman]

Currently libocmock_shared.dylib and libios_test_flutter.dylib are supposed to be embedded into IosUnitTests via a build phase bash script:

xcode_frameworks_dir="${TARGET_BUILD_DIR}/${PlugIns/IosUnitTestsTests.xctest/Frameworks/}"
mkdir -p "${xcode_frameworks_dir}"
flutter_engine_rel_path="../../../../out/${FLUTTER_ENGINE}"
cp -f "${flutter_engine_rel_path}/libios_test_flutter.dylib" "${xcode_frameworks_dir}"
cp -f "${flutter_engine_rel_path}/libocmock_shared.dylib" "${xcode_frameworks_dir}"
...

It's trying to copy these libraries to the .../Build/Products/Debug-iphonesimulator/IosUnitTests.app/PlugIns/IosUnitTestsTests.xctest/Frameworks directory. However, ${PlugIns/IosUnitTestsTests.xctest/Frameworks/} resolves to an empty string, so when it runs it's copying just to the build directory:

+ cp -f ../../../../out/ios_debug_sim_unopt/libios_test_flutter.dylib /Users/magder/Library/Developer/Xcode/DerivedData/IosUnitTests-deguovljqrrqvqcikqurolciwtuu/Build/Products/Debug-iphonesimulator/
+ cp -f ../../../../out/ios_debug_sim_unopt/libocmock_shared.dylib /Users/magder/Library/Developer/Xcode/DerivedData/IosUnitTests-deguovljqrrqvqcikqurolciwtuu/Build/Products/Debug-iphonesimulator/

Instead:

1. Replace the script phase with a copy phase that also handles code signing.
2. Adjust the test target's runpath search paths to find the embedded dylibs.

I found this while trying to get the unit tests running on a physical device, and noticed the dylibs weren't in the app bundle.
flutter/flutter#87699

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.