Correct YAML error in GHA + add macOS

- yamllint to the rescue. Still shows an error though:
  4:3       error    syntax error: expected <block end>, but found '<block mapping start>' (syntax)

- added macos, see https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners

.github/workflows/test.yml

@@ -1,24 +1,24 @@
 name: testssl.sh CI
 
 on: [push, pull_request]
-   paths-ignore:
-      - 'utils/**'
-      - 'docs/**'
-      - 'bin/**'
-      - '**.md'
-      - '**.pem'
-      - '**.pdf'
-      - '**.html'
-      - 'LICENSE'
-      - 'Dockerfile'
+  paths-ignore:
+    - 'utils/**'
+    - 'docs/**'
+    - 'bin/**'
+    - '**.md'
+    - '**.pem'
+    - '**.pdf'
+    - '**.html'
+    - 'LICENSE'
+    - 'Dockerfile'
 
 jobs:
   build:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: ['ubuntu-20.04']
-        perl: [ '5.26' ]
+        os: ['ubuntu-20.04', 'macos-10.15']
+        perl: ['5.26']
     name: Perl ${{ matrix.perl }} on ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2