Update the test suite so it uses the latest Docker image, and use Bourne shell for most tests, so weird shell issues are easier to catch.

Makefile

@@ -1,6 +1,6 @@
 SHELL := /usr/bin/env bash
 IMAGE ?= w0rp/ale
-CURRENT_IMAGE_ID = 82334204724d
+CURRENT_IMAGE_ID = 26d48d06e5e8
 DOCKER_FLAGS = --rm -v $(PWD):/testplugin -v $(PWD)/test:/home "$(IMAGE)"
 tests = test/*.vader test/*/*.vader test/*/*/*.vader test/*/*/*/*.vader