chore: add reproducible example of mocha missing async warning

v-next/example-project-assertion/.gitignore

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+# Node modules
+/node_modules
+
+# Compilation output
+/dist
+
+# pnpm deploy output
+/bundle
+
+# Hardhat Build Artifacts
+/artifacts
+
+# Hardhat compilation (v2) support directory
+/cache
+
+# Types generated by typechain
+/types
+
+# Ignition deployment output
+/ignition/deployments
+
+# Hardhat coverage reports
+/coverage

v-next/example-project-assertion/LICENSE

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+MIT License
+
+Copyright (c) 2024 Nomic Foundation
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

v-next/example-project-assertion/README.md

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+# Reproduction of mocha assertion error
+
+This is a deletable example project to reproduce an assertion error that 
+can happen with a missing await. The unpredictability of cleanup in a larger
+suite means this may not work for you.
+
+To run the example:
+
+```shell
+cd v-next/example-project-assertion
+pnpm hardhat test mocha
+
+# No contracts to compile
+
+# Running Mocha tests
+
+
+#   Hardhat3 test
+#     ✔ Should transfer money to another wallet with extra value (331ms)
+
+# Unhandled promise rejection:
+
+# HardhatError: HHE100: An internal invariant was violated: The block doesn't exist
+#     at assertHardhatInvariant (/workspaces/hardhat/v-next/hardhat-errors/src/errors.ts:237:11)
+#     at getBalanceChange (/workspaces/hardhat/v-next/hardhat-ethers-chai-matchers/src/internal/matchers/changeEtherBalance.ts:100:3)
+#     at async Promise.all (index 0)
+```
+
+There is also a test script under `./scripts/run-test-with-cleanup.js` that tries
+to recreate the issue by closing the connection - it is unsuccessful.