Add initial GitHub actions + replace AppVeyor

.github/workflows/test.yml

@@ -0,0 +1,43 @@
+name: Run all D Tests
+
+# Only triggers on pushes/PRs to master
+on:
+  pull_request:
+    branches:
+      - master
+  push:
+    branches:
+      - master
+      - github_actions
+
+jobs:
+    test:
+        name: Dub Tests
+        strategy:
+            matrix:
+                os: [ubuntu-latest, windows-latest, macOS-latest]
+                dc: [dmd-latest, ldc-latest]
+                arch: [x86_64]
+                include:
+                  - { os: windows-latest, dc: dmd-2.092.0, arch: x86_64 }
+                  - { os: windows-latest, dc: dmd-2.092.0, arch: x86 }
+                  - { os: windows-latest, dc: dmd-2.092.0, arch: x86_mscoff }
+                  - { os: windows-latest, dc: dmd-2.091.1, arch: x86_64 }
+                  - { os: windows-latest, dc: dmd-2.086.1, arch: x86_64 }
+                  - { os: windows-latest, dc: dmd-2.085.1, arch: x86_64 }
+                  - { os: windows-latest, dc: dmd-2.079.0, arch: x86_mscoff }
+                  - { os: windows-latest, dc: ldc-1.20.1, arch: x86_64 }
+                  - { os: windows-latest, dc: ldc-1.16.0, arch: x86_64 }
+                  - { os: windows-latest, dc: ldc-1.15.0, arch: x86_64 }
+
+        runs-on: ${{ matrix.os }}
+        steps:
+            - uses: actions/checkout@v2
+
+            - name: Install D compiler
+              uses: dlang-community/setup-dlang@v1
+              with:
+                  compiler: ${{ matrix.dc }}
+
+            - name: Run tests
+              run: dub test --arch ${{ matrix.arch }}