How to run MSTest tests using x64 configuration

You wrote some tests using MSTest for x64 architecture and they pass if you run them via Visual Studio test runner. But when you run them via ReSharper, you get this error even if you "force tests to run in 64-bit process" here ReSharper | Options | Tools | Unit Testing:

System.BadImageFormatException: Could not load file or assembly {some dll name} or one of its dependencies. An attempt was made to load a program with an incorrect format. 

The solution is the following: 

1. Set up .testsettings file (Open .testsettings | Hosts | Run Tests in 64 bit process on 64 bit machine) 
2. Point ReSharper to use it via ReSharper | Options | Unit Testing | MSTest | Select Test Run Configuration | Use this Test Run Configuration.