build: testbed GH workflow (first attempt)

.github/workflows/testbed.yml

@@ -0,0 +1,41 @@
+name: testbed
+
+on: [push]
+
+jobs:
+
+  linux:
+    timeout-minutes: 15
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Compile testbed
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTBED=ON ..
+        make -j4
+
+  windows:
+    timeout-minutes: 15
+    runs-on: windows-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Compile testbed
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTBED=ON ..
+        cmake --build . -j 4
+
+  macos:
+    timeout-minutes: 15
+    runs-on: macOS-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Compile testbed
+      working-directory: build
+      run: |
+        cmake -DENTT_BUILD_TESTBED=ON ..
+        make -j4