Merge fort-nix/nix-bitcoin#574: Add dev helper and docs

b4d7e1aa8f add dev helper and docs (Erik Arvstedt)
b35d08d3f2 docs: move test docs from `examples/README` to `test/README` (Erik Arvstedt)
4d76eb9183 docs/configuration: fix typo (Erik Arvstedt)
dc0710f3f4 tests: add example scenario `customTest` (Erik Arvstedt)
9e30d2728b tests: formatting (Erik Arvstedt)
c6d85c6fe3 tests: fix broken unit file when clightning is disabled (Erik Arvstedt)
a51f7b419e run-tests: use arg instead of env var for scenario overrides (Erik Arvstedt)

Pull request description:

ACKs for top commit:
  jonasnick:
    ACK b4d7e1aa8f

Tree-SHA512: f0ed8f8fe326c64eac3b7e9f48597dd00eedb9244333e199d18d1c2c06f369cd015f77aefd48e87235a68aee0b352057249525bf015e0a564fda380bdf7bb9d1

examples/README.md

@@ -51,60 +51,7 @@ the node:
 ```
 
 ### Tests
-The internal test suite is also useful for exploring features.\
-The following `run-tests.sh` commands leave no traces (outside of `/nix/store`) on
-the host system.
-
-`run-tests.sh` requires Nix >= 2.10.
-
-```bash
-git clone https://github.com/fort-nix/nix-bitcoin
-cd nix-bitcoin/test
-
-# Run a node in a VM. No tests are executed.
-./run-tests.sh vm
-systemctl status bitcoind
-
-# Run a Python test shell inside a VM node
-./run-tests.sh debug
-print(succeed("systemctl status bitcoind"))
-run_test("bitcoind")
-
-# Run a node in a container. Requires systemd and root privileges.
-./run-tests.sh container
-c systemctl status bitcoind
-
-# Explore a single feature
-./run-tests.sh --scenario electrs container
-
-# Run a command in a container
-./run-tests.sh --scenario '{
-  services.clightning.enable = true;
-  nix-bitcoin.nodeinfo.enable = true;
-}' container --run c nodeinfo
-```
-See [`run-tests.sh`](../test/run-tests.sh) for a complete documentation.
-
-#### Flakes
-Tests can also be directly accessed via Flakes:
-```bash
-# Build test
-nix build --no-link ..#tests.default
-
-# Run a node in a VM. No tests are executed.
-nix run ..#tests.default.vm
-
-# Run a Python test shell inside a VM node
-nix run ..#tests.default.run -- --debug
-
-# Run a node in a container. Requires extra-container, systemd and root privileges
-nix run ..#tests.default.container
-nix run ..#tests.default.containerLegacy # For NixOS with `system.stateVersion` <22.05
-
-# Run a command in a container
-nix run ..#tests.default.container -- --run c nodeinfo
-nix run ..#tests.default.containerLegacy -- --run c nodeinfo # For NixOS with `system.stateVersion` <22.05
-```
+The [nix-bitcoin test suite](../test/README.md) is also useful for exploring features.
 
 ### Real-world example
 Check the [server repo](https://github.com/fort-nix/nixbitcoin.org) for https://nixbitcoin.org