GH Actions/Test: Switch from self-hosted to standard runner

.github/workflows/test.yml

@@ -1,9 +1,20 @@
 name: Tests
-on: push
+on: [push, pull_request]
 jobs:
   test:
-    runs-on: self-hosted
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        julia-version: ['1.3', '1.4', '1']
+        julia-arch: [x64, x86]
+        os: [ubuntu-latest, windows-latest, macOS-latest]
+        exclude:
+          - os: macOS-latest
+            julia-arch: x86
     steps:
-    - uses: actions/checkout@v1
-    - name: Run unit tests
-      run: julia --project=@. -e 'using Pkg; Pkg.test("UnitCommitment")'
+      - uses: actions/checkout@v2
+      - uses: julia-actions/setup-julia@latest
+        with:
+          version: ${{ matrix.julia-version }}
+      - uses: julia-actions/julia-buildpkg@latest
+      - uses: julia-actions/julia-runtest@latest