Add a github action to run unit tests

We didn't have any CI until now, so this is the bare minimum. Using the
windows build agent as it has been updated to net5.0 (unlike linux,
which is [currently in
rollout](actions/runner-images#1891 (comment))).

Note that ppy#4 should be
merged first to fix a compilation error.

.github/workflows/test.yml

@@ -0,0 +1,17 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: windows-latest
+    name: Unit testing
+    steps:
+      - uses: actions/checkout@v1
+      - run: dotnet test