CI: Add MacOS

.github/workflows/tests.yml

diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 1ed2051..875104a 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -11,11 +11,13 @@
 
 jobs:
   build:
-
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: ['macos-10.15', 'ubuntu-latest']
+    runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@v2
     - run: ./script/install-bats.sh
     - run: git clone --depth 1 https://github.com/bats-core/bats-support ../bats-support
-    - run: bats test
+    - run: bats test
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