tests: optimize building multiple tests at once

The result of `import tests.nix {}` is now an attrset of tests.
This makes it easier and more efficient to evaluate or build multiple
tests in one call to `nix build`.

Simplify tests.nix by removing the large module args scope in favor of
self-contained scenario module definitions.

Add CPU core and memory size defaults to the test configuration to
enable building tests without `run-tests.sh`.

Add the following top-level args to tests.nix:
- `extraScenarios` to provide a nix-level way to define extra scenarios.
- `pkgs` to allow building tests with custom pkgs or systems.

test/lib/make-container.sh

@@ -84,6 +84,6 @@ if [[ ! ($containerBin && $(realpath $containerBin) == *extra-container-0.6*) ]]
 fi
 
 read -d '' src <<EOF || true
-(import "$scriptDir/tests.nix" { scenario = "$scenario"; }).container
+((import "$scriptDir/tests.nix" {}).getTest "$scenario").container
 EOF
 exec extra-container $containerCommand -E "$src" "$@"

test/lib/make-test-vm.nix

@@ -1,7 +1,7 @@
 pkgs:
 let
   pythonTesting = import "${toString pkgs.path}/nixos/lib/testing-python.nix" {
-    system = builtins.currentSystem;
+    system = pkgs.stdenv.hostPlatform.system;
     inherit pkgs;
   };
 in

test/lib/make-test.nix

@@ -1,7 +1,9 @@
+pkgs:
 let
-  pkgs =  import <nixpkgs> { config = {}; overlays = []; };
   makeVM = import ./make-test-vm.nix pkgs;
+  inherit (pkgs) lib;
 in
+
 name: testConfig:
 {
   vm = makeVM {
@@ -9,8 +11,15 @@ name: testConfig:
 
     machine = {
       imports = [ testConfig ];
-      # Needed because duplicity requires 270 MB of free temp space, regardless of backup size
-      virtualisation.diskSize = 1024;
+      virtualisation = {
+        # Needed because duplicity requires 270 MB of free temp space, regardless of backup size
+        diskSize = 1024;
+
+        # Min. 800 MiB needed to avoid 'out of memory' errors
+        memorySize = lib.mkDefault 2048;
+
+        cores = lib.mkDefault 2;
+      };
     };
 
     testScript = nodes: let