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.
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