Tests behaviour for SerialPortStream.

With .NET Core 3.1 the ReadAsync call blocks while waiting for reading from the COM port. Even if we WriteAsync and FlushAsync, nothing gets read from the COM port and the ReadAsync blocks. It works for .NET 4.7.2.

The unit test project sets up the two target frameworks .NET 4.7.2 and .NET Core 3.1. All tests will be run for both frameworks.