Run tests on all Windows versions

Some time ago GitHub has introduced an alternative version of Windows
virtual environment. One of those is possibly untested in this repo.
Let's test both of them.

Audit tasks and side tracks are still using "windows-latest" because we
don't particularly care about them. However, main tests should be run
against both versions.

You can find currently supported environments here:
https://github.com/actions/virtual-environments/

.github/workflows/main.yml

@@ -12,7 +12,10 @@ on:
 jobs:
   test:
     name: default
-    runs-on: windows-latest
+    runs-on: ${{ matrix.windows }}
+    strategy:
+      matrix:
+        windows: [windows-2016, windows-2019]
     steps:
       - name: Check out source code
         uses: actions/checkout@v2

.github/workflows/release.yml

@@ -10,7 +10,10 @@ on:
 jobs:
   test:
     name: release
-    runs-on: windows-latest
+    runs-on: ${{ matrix.windows }}
+    strategy:
+      matrix:
+        windows: [windows-2016, windows-2019]
     steps:
       - name: Setup Developer Command Prompt
         uses: ilammy/msvc-dev-cmd@v1