ci: have at least one CTest run in parallel per platform

...this help detect race conditions between tests, besides speeding up
test

Author: scivision

.github/workflows/CI.yml

@@ -55,10 +55,15 @@ jobs:
       run: cmake -Wdev -DCMAKE_BUILD_TYPE=Release -S . -B build
 
     - name: Build and compile
-      run: cmake --build build || cmake --build build --verbose --parallel 1
+      run: cmake --build build
+
+    - run: cmake --build build --verbose --parallel 1
+      if: failure()
 
     - name: test
-      run: cmake --build build --target test
+      run: ctest --parallel
+      working-directory: build
+
 
     - name: Test in-tree builds
       if: contains( matrix.gcc_v, '9') # Only test one compiler on each platform