Enable testing on .NET Core 2.1

Author: slozier

.github/workflows/main.yml

@@ -19,9 +19,14 @@ jobs:
     - uses: actions/checkout@v2
       with:
         submodules: true
-    - uses: actions/setup-dotnet@v1
+    - name: Setup .NET Core 2.1
+      uses: actions/setup-dotnet@main
       with:
-        dotnet-version: '3.1.302'
+        dotnet-version: '2.1.810'
+    - name: Setup .NET Core 3.1
+      uses: actions/setup-dotnet@main
+      with:
+        dotnet-version: '3.1.402'
     - name: Set Mono Version
       if: matrix.os == 'macos-latest'
       run: echo ::add-path::/Library/Frameworks/Mono.framework/Versions/6.4.0/bin
@@ -42,8 +47,6 @@ jobs:
       run: ./make.ps1 -frameworks net46 test-all
       shell: pwsh
     - name: Test (netcoreapp2.1)
-      # TODO: figure out how to test on other OSs
-      if: matrix.os == 'windows-latest'
       run: ./make.ps1 -frameworks netcoreapp2.1 test-all
       shell: pwsh
     - name: Test (netcoreapp3.1)