Add more tests (#253)

I'm not entirely sure how useful all of these will be yet - still
trying to work out what/when to test things, but hopefully this'll
be a useful datapoint.

src/test/resources/test-rom/spec/programs/edit_spec.lua

@@ -1,10 +1,9 @@
 local capture = require "test_helpers".capture_program
+local testFile = require "test_helpers".testFile
 
 describe("The edit program", function()
 
-    it("displays its usage when given no argument", function()
-        multishell = nil
-        
+    it("displays its usage when given no argument", function()      
         expect(capture(stub, "edit"))
             :matches { ok = true, output = "Usage: edit <path>\n", error = "" }
     end)