[Tests] 升级至 net 5.0

tests/Essensoft.AspNetCore.Payment.Alipay.Tests/Essensoft.AspNetCore.Payment.Alipay.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net5.0</TargetFramework>
     <IsPackable>false</IsPackable>
   </PropertyGroup>