use netcoreapp2.1 for testing

appveyor.bat

@@ -38,7 +38,7 @@ dotnet restore
 dotnet build -c %configuration% || goto error
 
 echo Testing C#
-dotnet test -c %configuration% -f netcoreapp1.0 Google.Protobuf.Test\Google.Protobuf.Test.csproj || goto error
+dotnet test -c %configuration% -f netcoreapp2.1 Google.Protobuf.Test\Google.Protobuf.Test.csproj || goto error
 dotnet test -c %configuration% -f net451 Google.Protobuf.Test\Google.Protobuf.Test.csproj || goto error
 
 goto :EOF

csharp/buildall.bat

@@ -4,7 +4,7 @@ dotnet build src/Google.Protobuf.sln || goto :error
 
 echo Running tests.
 
-dotnet test -f netcoreapp1.0 src/Google.Protobuf.Test/Google.Protobuf.Test.csproj || goto :error
+dotnet test -f netcoreapp2.1 src/Google.Protobuf.Test/Google.Protobuf.Test.csproj || goto :error
 
 goto :EOF
 

csharp/buildall.sh

@@ -10,8 +10,8 @@ dotnet restore $SRC/Google.Protobuf.sln
 dotnet build -c $CONFIG $SRC/Google.Protobuf.sln
 
 echo Running tests.
-# Only test netcoreapp1.0, which uses the .NET Core runtime.
+# Only test netcoreapp2.1, which uses the .NET Core runtime.
 # If we want to test the .NET 4.5 version separately, we could
 # run Mono explicitly. However, we don't have any differences between
-# the .NET 4.5 and netstandard1.0 assemblies.
-dotnet test -c $CONFIG -f netcoreapp1.0 $SRC/Google.Protobuf.Test/Google.Protobuf.Test.csproj
+# the .NET 4.5 and netstandard2.1 assemblies.
+dotnet test -c $CONFIG -f netcoreapp2.1 $SRC/Google.Protobuf.Test/Google.Protobuf.Test.csproj