test: add common.mustCall function

Verifies that the callback gets invoked <n> times during the lifetime of the
test script.

This is a back-port of commit d0e6c3f from the master branch.

test/common.js

@@ -139,8 +139,43 @@ process.on('exit', function() {
 });
 
 
-// This function allows one two run an HTTP test agaist both HTTPS and
-// normal HTTP modules. This ensures they fit the same API.
-exports.httpTest = function httpTest(cb) {
+var mustCallChecks = [];
+
+
+function runCallChecks() {
+  var failed = mustCallChecks.filter(function(context) {
+    return context.actual !== context.expected;
+  });
+
+  failed.forEach(function(context) {
+    console.log('Mismatched %s function calls. Expected %d, actual %d.',
+                context.name,
+                context.expected,
+                context.actual);
+    console.log(context.stack.split('\n').slice(2).join('\n'));
+  });
+
+  if (failed.length) process.exit(1);
+}
+
+
+exports.mustCall = function(fn, expected) {
+  if (typeof expected !== 'number') expected = 1;
+
+  var context = {
+    expected: expected,
+    actual: 0,
+    stack: (new Error).stack,
+    name: fn.name || '<anonymous>'
   };
 
+  // add the exit listener only once to avoid listener leak warnings
+  if (mustCallChecks.length === 0) process.on('exit', runCallChecks);
+
+  mustCallChecks.push(context);
+
+  return function() {
+    context.actual++;
+    return fn.apply(this, arguments);
+  };
+};