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.

test/clightning-replication.nix

@@ -1,12 +1,9 @@
 # You can run this test via `run-tests.sh -s clightningReplication`
 
-let
-  nixpkgs = (import ../pkgs/nixpkgs-pinned.nix).nixpkgs;
-in
-import "${nixpkgs}/nixos/tests/make-test-python.nix" ({ pkgs, ... }:
+makeTestVM: pkgs:
 with pkgs.lib;
 let
-  keyDir = nixpkgs + "/nixos/tests/initrd-network-ssh";
+  keyDir = pkgs.path + "/nixos/tests/initrd-network-ssh";
   keys = {
     server = keyDir + "/ssh_host_ed25519_key";
     client = keyDir + "/id_ed25519";
@@ -29,7 +26,7 @@ let
     };
   };
 in
-{
+makeTestVM {
   name = "clightning-replication";
 
   nodes = let nodes = {
@@ -150,4 +147,4 @@ in
           # A gocryptfs has been created on the server
           server.succeed("ls /var/backup/nb-replication/writable/lightningd-db/gocryptfs.conf")
   '';
-})
+}