ci(runner): Run tests on specific Ubuntu versions

Factors out runner OS into separate workflows that each call a shared
test matrix workflow. This allows CI status badges for each runner OS,
rather than a single status badge for all OSes.

Having a CI status badge for each supported runner OS allows
at-a-glance judgement of this Action's maintenance status.

Resolves: #106

.github/workflows/test-ubuntu-18.04.yml

@@ -0,0 +1,20 @@
+name: ubuntu-18.04
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  tests:
+    name: ubuntu-18.04
+    uses: ./.github/workflows/tests.yml
+    with:
+      runner-os: "ubuntu-18.04"

.github/workflows/test-ubuntu-20.04.yml

@@ -0,0 +1,20 @@
+name: ubuntu-20.04
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  tests:
+    name: ubuntu-20.04
+    uses: ./.github/workflows/tests.yml
+    with:
+      runner-os: "ubuntu-20.04"

.github/workflows/test-ubuntu-22.04.yml

@@ -0,0 +1,20 @@
+name: ubuntu-22.04
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  tests:
+    name: ubuntu-22.04
+    uses: ./.github/workflows/tests.yml
+    with:
+      runner-os: "ubuntu-22.04"

.github/workflows/tests.yml

@@ -1,25 +1,24 @@
 name: Tests
 
 on:
-  push:
-    branches:
-      - main
-  pull_request:
-    branches:
-      - main
+  workflow_call:
+    inputs:
+      runner-os:
+        required: true
+        type: string
 
 defaults:
   run:
     shell: bash
 
 jobs:
   test:
-    name: clang-format-${{ matrix.version }}
-    runs-on: ubuntu-latest
+    name: clang-format-${{ matrix.clang-format-version }}
+    runs-on: ${{ inputs.runner-os }}
     strategy:
       fail-fast: false
       matrix:
-        version:
+        clang-format-version:
           - 3
           - 4
           - 5
@@ -35,4 +34,4 @@ jobs:
     steps:
     - uses: actions/checkout@v3
     - name: Build and test the Docker image
-      run: test/test.sh ${{ matrix.version }}
+      run: test/test.sh ${{ matrix.clang-format-version }}