Run the pong timeout tests in the main test suite

696b94b built and ran test_websocket_heartbeat in the Windows jobs to
cover the pong timeout wake, which added more than a minute to each of
them. Only its two pong timeout tests exercise what differs by
platform, and both work with the ping interval set at runtime. Move
them into test.cc, where every job runs them in its shards, and drop
the Windows heartbeat build and step.

The heartbeat file keeps CompileTimeClientReadTimeoutIsFail, whose
fixture is renamed WebSocketSilentPeerTest after the peer it sets up.

.github/workflows/test.yaml

@@ -623,10 +623,6 @@ jobs:
         }
         if ($failed) { exit 1 }
         Write-Host "All shards passed."
-    - name: Run WebSocket heartbeat test ${{ matrix.config.name }}
-      if: ${{ matrix.config.run_tests }}
-      working-directory: build/test
-      run: ./Release/httplib-heartbeat-test.exe --gtest_color=yes
 
     env:
       VCPKG_ROOT: "C:/vcpkg"