test: allow for testing all scenarios

Test all scenarios by default when running 'build' (which happens
when the script is called without arguments).

Default to scenario 'default' in other test commands like 'debug'.

test/test.nix

@@ -1,5 +1,6 @@
 # Integration test, can be run without internet access.
 
+# Make sure to update build() in ./run-tests.sh when adding new scenarios
 { scenario ? "default" }:
 
 import ./make-test.nix rec {