tests(spread): test snap parallel installs

tests/spread/smoketests/parallel-install/charmcraft.yaml

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+type: charm
+name: test-charm
+summary: test-charm
+description: test-charm
+
+base: ubuntu@24.04
+platforms:
+  amd64:
+  arm64:
+  riscv64:
+
+parts:
+  my-part:
+    plugin: nil

tests/spread/smoketests/parallel-install/task.yaml

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+summary: pack a charm with parallel-installed charmcraft versions
+
+# Run last since we change snapd settings
+priority: -10
+
+prepare: |
+  snap install yq
+  if [[ $(snap get system experimental.parallel-instances) != true ]]; then
+    snap set system experimental.parallel-instances=true
+    REBOOT
+  fi
+  snap install --classic --channel=latest/candidate charmcraft
+  snap install --dangerous --classic --name=charmcraft_dev /charmcraft/charmcraft_*.snap
+
+restore: |
+  if [[ $(snap get system experimental.parallel-instances) == true ]]; then
+    snap remove charmcraft_dev
+    snap set system experimental.parallel-instances=false
+    REBOOT
+  fi
+  snap install --classic --dangerous /charmcraft/charmcraft_*.snap
+
+execute: |
+  # Check that the candidate version used the correct version
+  charmcraft pack
+  [[ $(unzip -p *.charm manifest.yaml | yq .charmcraft-version) == $(charmcraft --version | cut -f2 -d' ') ]]
+  rm *.charm
+
+  # Try the dev version
+  charmcraft_dev pack
+  [[ $(unzip -p *.charm manifest.yaml | yq .charmcraft-version) == $(charmcraft_dev --version | cut -f2 -d' ') ]]