docs: add testing

TESTING.md

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+# Testing on testnet
+
+## Creating a gift wallet
+
+[Download node-funder](https://github.com/ethersphere/node-funder) which will be used to create gift wallets.
+
+Make sure your Go version is 1.18 or above. Run `go version` to verify.
+
+Set some environment variables:
+
+```bash
+export TESTNET_PRIVATE_KEY="..."
+export TESTNET_RPC_ENDPOINT="https://goerli.infura.io/v3/..."
+export NODE_FUNDER_PATH="<path to node-funder>"
+```
+
+Make sure you have the latest utility CLI version: `npm install --global cafe-tui`
+
+Create a gift wallet with the command `npx cafe-tui funder gift` and make notes as it is not stored anywhere.
+
+## Switching Bee to testnet
+
+Overwrite the following values in `config.yaml` which is at `~/Library/Application Support/Swarm Desktop`:
+
+```yaml
+mainnet: false
+data-dir: 'data-dir-testnet'
+swap-enable: false
+swap-endpoint: ''
+resolver-options: 'https://goerli.infura.io/v3/...'
+bootnode: '/dnsaddr/testnet.ethswarm.org
+```
+
+## Getting new Bee
+
+Download the Bee binary under test from the [releases page](https://github.com/ethersphere/bee/releases)
+
+Rename it to `bee` and move it to `~/Library/Application Support/Swarm Desktop`
+
+## Running Desktop
+
+Grab `swarm-desktop` source and run the following:
+
+```bash
+cd ui && npm install
+cd .. && npm install
+npm start
+```
+
+## Switching Dashboard to testnet
+
+Grab `bee-dashboard` source and switch to branch `chore/testnet`. It contains the testnet `BZZ_TOKEN_ADDRESS` and
+`NETWORK_ID`.
+
+Start it with `REACT_APP_BEE_DESKTOP_URL=http://localhost:3054 REACT_APP_BEE_DESKTOP_ENABLED=true npm start`
+
+Open URL `http://localhost:3002/?v=YOUR_API_KEY` with your Desktop API key. You may use the following command on OSX:
+
+```bash
+open "http://localhost:3002/?v=$(cat ~/Library/Application\ Support/Swarm\ Desktop/api-key.txt)"
+```