Add a test utility for capturing program output

- Make mcfly's stubbing system a little more fault-tolerant.
 - Add a small utility function which redirects print, printError and
   write to capture their output, rather than printing to the terminal.
   This can then be matched against in order to determine a program's
   output.
   It's a little flakey - you can't use it multiple times in an it
   block, etc... but it's a nice feature.
 - Add a small couple of tests to delete as a proof-of-concept.

src/test/resources/test-rom/mcfly.lua

@@ -34,10 +34,12 @@ local active_stubs = {}
 -- @tparam string var The variable to stub
 -- @param value The value to stub it with
 local function stub(tbl, var, value)
-    table.insert(active_stubs, { tbl = tbl, var = var, value = tbl[var] })
-    _G[var] = value
-end
+    check('stub', 1, 'table', tbl)
+    check('stub', 2, 'string', var)
 
+    table.insert(active_stubs, { tbl = tbl, var = var, value = tbl[var] })
+    rawset(tbl, var, value)
+end
 
 --- Capture the current global state of the computer
 local function push_state()
@@ -55,7 +57,7 @@ end
 local function pop_state(state)
     for i = #active_stubs, 1, -1 do
         local stub = active_stubs[i]
-        stub.tbl[stub.var] = stub.value
+        rawset(stub.tbl, stub.var, stub.value)
     end
 
     active_stubs = state.stubs
@@ -353,6 +355,9 @@ if not fs.isDir(root_dir) then
     error()
 end
 
+-- Ensure the test folder is also on the package path
+package.path = ("/%s/?.lua;/%s/?/init.lua;%s"):format(root_dir, root_dir, package.path)
+
 do
     -- Load in the tests from all our files
     local env = setmetatable({}, { __index = _ENV })

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

@@ -1,3 +1,5 @@
+local capture = require "test_helpers".capture_program
+
 describe("The rm program", function()
     local function touch(file)
         io.open(file, "w"):close()
@@ -32,4 +34,14 @@ describe("The rm program", function()
         expect(fs.exists("/test-files/a.txt")):eq(false)
         expect(fs.exists("/test-files/b.txt")):eq(false)
     end)
+
+    it("displays the usage with no arguments", function()
+        expect(capture(stub, "rm"))
+            :matches { ok = true, output = "Usage: rm <paths>\n", error = "" }
+    end)
+
+    it("errors when a glob fails to match", function()
+        expect(capture(stub, "rm", "never-existed"))
+            :matches { ok = true, output = "", error = "never-existed: No matching files\n" }
+    end)
 end)

src/test/resources/test-rom/spec/test_helpers.lua

@@ -0,0 +1,46 @@
+--- Run a program and capture its output
+--
+-- @tparam function(tbl:table, var:string, value:string) stub The active stub function.
+-- @tparam string program The program name.
+-- @tparam string ... Arguments to this program.
+-- @treturn { ok = boolean, output = string, error = string, combined = string }
+-- Whether this program terminated successfully, and the various output streams.
+local function capture_program(stub, program, ...)
+    local output, error, combined = {}, {}, {}
+
+    local function out(stream, msg)
+        table.insert(stream, msg)
+        table.insert(combined, msg)
+    end
+
+    stub(_G, "print", function(...)
+        for i = 1, select('#', ...) do
+            if i > 1 then out(output, " ") end
+            out(output, tostring(select(i, ...)))
+        end
+        out(output, "\n")
+    end)
+
+    stub(_G, "printError", function(...)
+        for i = 1, select('#', ...) do
+            if i > 1 then out(error, " ") end
+            out(error, tostring(select(i, ...)))
+        end
+        out(error, "\n")
+    end)
+
+    stub(_G, "write", function(msg) out(output, tostring(msg)) end)
+
+    local ok = shell.run(program, ...)
+
+    return {
+        output = table.concat(output),
+        error = table.concat(error),
+        combined = table.concat(combined),
+        ok = ok
+    }
+end
+
+return {
+    capture_program = capture_program,
+}