testsuite driver stuck infinitely when testing the wasm backend with normal speed

When testing the wasm backend with normal speed, the testsuite driver process is stuck infinitely with 100% CPU occupation rate after all test cases are run.

export CROSS_EMULATOR=${CROSS_EMULATOR:-"/workspace/.ghc-wasm/wasm-run/bin/wasmtime.sh"}
hadrian/build \
  --no-color \
  --no-progress \
  --no-time \
  --flavour=perf \
  --docs=none \
  -j \
  test \
  --summary-junit=junit.xml \
  --summary=summary.txt \
  --test-speed=normal \
  --skip-perf

@bgamari suggests it may be an interpreter livelock. Though it's stuck even if the driver is executed by pypy3, so maybe the multi-threading logic in the driver is broken in a certain way that's somehow triggered by the wasm backend.

It seems to me that rewriting a small part of the driver that deals with concurrency may be sufficient to make things more robust and workaround this bug.