test: improve modularization

This improves debugging and experimenting by making it easy to compose fine-grained
scenarios that have specific tests and features enabled.

The VM test output now includes the subtest name and duration.

Remove the 'raise Exception()' hack for interactive mode.

Run 'banlist-and-restart' test before 'backups'. This speeds up the test
by avoiding an extra shutdown of all bitcoin-related services.

test/lib/make-test-vm.nix

@@ -0,0 +1,35 @@
+testArgs:
+
+let
+  pkgs = import <nixpkgs> { config = {}; overlays = []; };
+
+  pkgs19_09 = import (pkgs.fetchzip {
+    url = "https://github.com/NixOS/nixpkgs-channels/archive/a7ceb2536ab11973c59750c4c48994e3064a75fa.tar.gz";
+    sha256 = "0hka65f31njqpq7i07l22z5rs7lkdfcl4pbqlmlsvnysb74ynyg1";
+  }) { config = {}; overlays = []; };
+
+  test = (import "${pkgs.path}/nixos/tests/make-test-python.nix") testArgs;
+
+  fixedTest = { system ? builtins.currentSystem, ... }@args:
+    let
+       pkgsFixed = pkgs // {
+         # Fix the black Python code formatter that's used in the test to allow the test
+         # script to have longer lines. The default width of 88 chars is too restrictive for
+         # our script.
+         python3Packages = pkgs.python3Packages // {
+           black = pkgs.writeScriptBin "black" ''
+             fileToCheck=''${@:$#}
+             [[ $fileToCheck = *test-script ]] && extraArgs='--line-length 100'
+             exec ${pkgs.python3Packages.black}/bin/black $extraArgs "$@"
+           '';
+         };
+
+         # QEMU 4.20 from unstable fails on Travis build nodes with message
+         # "error: failed to set MSR 0x48b to 0x159ff00000000"
+         # Use version 4.0.1 instead.
+         inherit (pkgs19_09) qemu_test;
+       };
+    in
+      test (args // { pkgs = pkgsFixed; });
+in
+  fixedTest