Upgrade test project to .net core 3.1

Author: lilith

tests/Imageflow.Test/Imageflow.Test.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <IsPackable>false</IsPackable>
     <Platforms>AnyCPU;x64;x86</Platforms>
     <LangVersion>7.1</LangVersion>