Add unit tests for specified areas

Is your feature request related to a problem? Please describe.
There are several cases covered by functional tests, which are more suitable for unit tests. Functional tests are much slower, so convert the appropriate ones to unit tests.

Describe the solution you'd like
Have more unit tests for scenarios currently covered with functional tests. Delete the affected functional tests after that. This will speed up the build.

Describe alternatives you've considered
No alternatives.

Additional context
Add unit tests for the following area:

• Merging of build.xcconfig files:
  • Test scenario with build.xcconfig from plugin/multiple plugins
  • Test scenario with build.xcconfig from App_Resources/iOS
  • Test scenario with build.xcconfig from App_Resources/iOS and from plugins simultaneously

Tests related to work with devices

• Test to check More than one device found. is raised when we have more than once device and command wants to specify exact device (for example tns deploy).
  Reference: https://github.com/NativeScript/nativescript-tooling-qa/blob/b159533e42602c7a94f5ae349036d15014b9a834/tests/cli/device/test_deploy_and_device.py#L143

• Test to check how we handle invalid device id passed with --device.
  Reference: https://github.com/NativeScript/nativescript-tooling-qa/blob/b159533e42602c7a94f5ae349036d15014b9a834/tests/cli/device/test_deploy_and_device.py#L164

Tests related to passing invalid platform to commands

• Tests to check how we handle passing wrong platform to commands like tns deploy and tns device.
  Reference:
  https://github.com/NativeScript/nativescript-tooling-qa/blob/b159533e42602c7a94f5ae349036d15014b9a834/tests/cli/device/test_deploy_and_device.py#L148
  https://github.com/NativeScript/nativescript-tooling-qa/blob/master/tests/cli/device/test_deploy_and_device.py#L160
• tns platform list with wrong project path
• tns platform add existing platform
• tns platform add wrong option
• tns platform remove missing invalid or empty_platform
• tns platform update empty platform

Tests related to signing behavior for iOS platform

• tns run ios --emulator and tns run ios --provision <provision> should rebuild the project
• tns run ios --provision <provision> and tns run ios --provision <provision> shouldn't rebuild the project
• tns run ios --emulator and tns run ios --emulator shouldn't rebuild the project

Tests for ios-project-service

• Adding .framework and .xcframeworks to Xcode project (preparePluginNativeCode)