tests: define tests via flake

Advantages:
- Pure test evaluations
- The test framework can now be used by flakes that extend nix-bitcoin
- Most features of `run-tests.sh` are now accessible via `nix build`/`nix run`.
  We keep `run-tests.sh` for advanced features like `scenarioOverridesFile` and adhoc scenarios.

Other changes:
- `run-tests.sh` now builds aggregate VM tests like `basic` or
  `buildable` by creating all VMs in a single evaluation.
  This speeds up the tests and eases debugging by separating the eval and build steps.
- Use the new `nix` CLI which has improved build output logging
  by prefixing output lines with the origin drv name.

flake.lock

@@ -1,5 +1,27 @@
 {
   "nodes": {
+    "extra-container": {
+      "inputs": {
+        "flake-utils": [
+          "flake-utils"
+        ],
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1666443795,
+        "owner": "erikarvstedt",
+        "repo": "extra-container",
+        "rev": "3b69ecfd363983cdee4db7f5d118b0ca099d23ed",
+        "type": "github"
+      },
+      "original": {
+        "owner": "erikarvstedt",
+        "repo": "extra-container",
+        "type": "github"
+      }
+    },
     "flake-utils": {
       "locked": {
         "lastModified": 1659877975,
@@ -49,6 +71,7 @@
     },
     "root": {
       "inputs": {
+        "extra-container": "extra-container",
         "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs",
         "nixpkgs-unstable": "nixpkgs-unstable"