test/busted: eager-flushing TAP.lua handler

This TAP.lua implementation was upstreamed:
f9db364291
but it has not been released yet. It avoids hangs, and helps debug
hangs, on QuickBuild, so inline it until the next busted release.

busted/outputHandlers/TAP.lua

@@ -0,0 +1,92 @@
+-- TODO(jkeyes): remove this and use the upstream version as soon as it is
+-- available in a release of busted.
+
+local pretty = require 'pl.pretty'
+
+return function(options)
+  local busted = require 'busted'
+  local handler = require 'busted.outputHandlers.base'()
+
+  local success = 'ok %u - %s'
+  local failure = 'not ' .. success
+  local skip = 'ok %u - # SKIP %s'
+  local counter = 0
+
+  handler.suiteReset = function()
+    counter = 0
+    return nil, true
+  end
+
+  handler.suiteEnd = function()
+    print('1..' .. counter)
+    io.flush()
+    return nil, true
+  end
+
+  local function showFailure(t)
+    local message = t.message
+    local trace = t.trace or {}
+
+    if message == nil then
+      message = 'Nil error'
+    elseif type(message) ~= 'string' then
+      message = pretty.write(message)
+    end
+
+    print(failure:format(counter, t.name))
+    print('# ' .. t.element.trace.short_src .. ' @ ' .. t.element.trace.currentline)
+    if t.randomseed then print('# Random seed: ' .. t.randomseed) end
+    print('# Failure message: ' .. message:gsub('\n', '\n# '))
+    if options.verbose and trace.traceback then
+      print('# ' .. trace.traceback:gsub('^\n', '', 1):gsub('\n', '\n# '))
+    end
+  end
+
+  handler.testStart = function(element, parent)
+    local trace = element.trace
+    if options.verbose and trace and trace.short_src then
+      local fileline = trace.short_src .. ' @ ' ..  trace.currentline .. ': '
+      local testName = fileline .. handler.getFullName(element)
+      print('# ' .. testName)
+    end
+    io.flush()
+
+    return nil, true
+  end
+
+  handler.testEnd = function(element, parent, status, trace)
+    counter = counter + 1
+    if status == 'success' then
+      local t = handler.successes[#handler.successes]
+      print(success:format(counter, t.name))
+    elseif status == 'pending' then
+      local t = handler.pendings[#handler.pendings]
+      print(skip:format(counter, (t.message or t.name)))
+    elseif status == 'failure' then
+      showFailure(handler.failures[#handler.failures])
+    elseif status == 'error' then
+      showFailure(handler.errors[#handler.errors])
+    end
+    io.flush()
+
+    return nil, true
+  end
+
+  handler.error = function(element, parent, message, debug)
+    if element.descriptor ~= 'it' then
+      counter = counter + 1
+      showFailure(handler.errors[#handler.errors])
+    end
+    io.flush()
+
+    return nil, true
+  end
+
+  busted.subscribe({ 'suite', 'reset' }, handler.suiteReset)
+  busted.subscribe({ 'suite', 'end' }, handler.suiteEnd)
+  busted.subscribe({ 'test', 'start' }, handler.testStart, { predicate = handler.cancelOnPending })
+  busted.subscribe({ 'test', 'end' }, handler.testEnd, { predicate = handler.cancelOnPending })
+  busted.subscribe({ 'error' }, handler.error)
+
+  return handler
+end