Don't use llvm-cov for cts_runner tests

Author: andyleiserson

.github/workflows/cts.yml

@@ -99,9 +99,12 @@ jobs:
         if: matrix.os == 'ubuntu-24.04'
         uses: ./.github/actions/install-mesa
 
+      # Mixing --llvm-cov with deno (rusty_v8) has linking problems.
+      # Explicitly set the backend to avoid EGL messages in output,
+      # because these tests check stdout.
       - name: Test cts_runner
         shell: bash
-        run: cargo --locked xtask test --llvm-cov -p cts_runner
+        run: DENO_WEBGPU_BACKEND=${{ matrix.backend }} cargo --locked test -p cts_runner
 
       - name: Run CTS
         shell: bash