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" "$@"