CI: raise cross-run timeouts & use all cores

On Ubuntu 22.04, these tests take significantly longer than they did on
20.04 for reasons not (yet) known.  In lieu of understanding, try
raising the timeouts and taking advantage of both cores in the
github-hosted runner VMs.

.github/workflows/main.yml

@@ -237,8 +237,8 @@ jobs:
     # QEMU)
     - name: Test
       working-directory: ${{github.workspace}}/build
-      run: ctest --output-on-failure -E '(perf-.*)|(.*-malloc$)' --timeout 400
-      timeout-minutes: 30
+      run: ctest -j 2 --output-on-failure -E '(perf-.*)|(.*-malloc$)' --timeout 400
+      timeout-minutes: 45
 
   windows:
     strategy: