limit parallelism based on available memory too

.forgejo/workflows/test.yml

@@ -59,7 +59,7 @@ jobs:
           make -C sby install
       - run: |
           git clone --depth=1 --recursive --branch=0.45 https://github.com/YosysHQ/yosys.git
-          make -C yosys -j2
+          make -C yosys -j"$(nproc)"
           make -C yosys install
       - run: |
           wget -O firrtl.tar.gz https://github.com/llvm/circt/releases/download/firtool-1.86.0/firrtl-bin-linux-x64.tar.gz
@@ -70,4 +70,12 @@ jobs:
       - uses: https://github.com/Swatinem/rust-cache@v2
         with:
           save-if: ${{ github.ref == 'refs/heads/master' }}
-      - run: cargo test
+      - run: |
+          avail_mem="$(free -b | awk '/^Mem:/{print($2)}')"
+          # estimate that each test needs 16GiB
+          ((par_from_mem = avail_mem / (16 << 30)))
+          par="$(nproc)"
+          if ((par_from_mem > 0 && par > par_from_mem)); then
+              par="$par_from_mem"
+          fi
+          cargo test -- --test-threads="$par"