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/cd_spec.lua

@@ -1,19 +1,18 @@
 local capture = require "test_helpers".capture_program
 
 describe("The cd program", function()
-
-    it("cd into a directory", function()
-        shell.run("cd /rom/programs")
-        
-        expect(shell.dir()):eq("rom/programs")
+    it("changes into a directory", function()
+        local setDir = stub(shell, "setDir")
+        capture(stub, "cd /rom/programs")
+        expect(setDir):called_with("rom/programs")
     end)
 
-    it("cd into a not existing directory", function()
+    it("does not move into a non-existent directory", function()
         expect(capture(stub, "cd /rom/nothing"))
             :matches { ok = true, output = "Not a directory\n", error = "" }
     end)
-    
-    it("displays the usage with no arguments", function()
+
+    it("displays the usage when given no arguments", function()
         expect(capture(stub, "cd"))
             :matches { ok = true, output = "Usage: cd <path>\n", error = "" }
     end)