Fix duplication of Info.plist copying that was causing Xcode errors

[igor-makarov]

Under Xcode 9 new build system, the dependency tree checker is much more strict.
If an Info.plist appears both as the target's .plist file, and in the build resource phase, Xcode will halt with an error similar to this:

Multiple commands produce '/Users/Igor/Library/Developer/Xcode/DerivedData/Project-afqnptmawwntueehujlkvjtarbgk/Build/Products/Debug-iphonesimulator/BananaApp.app/Info.plist':
1) Target 'BananaApp' (project 'Project') has copy command from '/Users/Igor/xcake/spec/fixtures/integration/simple_with_info_plist/temp/BananaApp/Supporting Files/Info.plist' to '/Users/Igor/Library/Developer/Xcode/DerivedData/Project-afqnptmawwntueehujlkvjtarbgk/Build/Products/Debug-iphonesimulator/BananaApp.app/Info.plist'
2) Target 'BananaApp' (project 'Project') has process command with output '/Users/Igor/Library/Developer/Xcode/DerivedData/Project-afqnptmawwntueehujlkvjtarbgk/Build/Products/Debug-iphonesimulator/BananaApp.app/Info.plist'

This PR aims to solve the issue by exempting a file if it's the target's plist file.

I've added unit tests for the case, and, for the first time, an integration test that does the following:

• Verifies that the generated .pbxproj matches the fixture by using Xcodeproj::Differ.project_diff
• Runs xcodebuild and makes sure it passes
• Verifies that the xcodebuild output matches the fixture