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-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