fix required test files caching, closes mochajs#736

Makefile

@@ -51,7 +51,8 @@ test-browser-tdd:
 
 test-jsapi:
 	@printf "==> [Test :: JS API]\n"
-	node test/jsapi
+	node test/jsapi/index.js
+	node test/jsapi/cache/index.js
 
 test-unit:
 	@printf "==> [Test :: Unit]\n"

lib/mocha.js

@@ -99,6 +99,10 @@ function Mocha(options) {
   if (options.slow) {
     this.slow(options.slow);
   }
+
+  // files which were in require.cache
+  // by the moment of mocha instance creation
+  this._requireCache = this._getRequireCache();
 }
 
 /**
@@ -220,6 +224,8 @@ Mocha.prototype.loadFiles = function(fn) {
     suite.emit('require', require(file), file, self);
     suite.emit('post-require', global, file, self);
   });
+
+  this._flushRequireCache();
   fn && fn();
 };
 
@@ -246,6 +252,28 @@ Mocha.prototype._growl = function(runner, reporter) {
   });
 };
 
+/**
+ * Get require.cache files (node only)
+ *
+ * @api private
+ */
+Mocha.prototype._getRequireCache = function() {
+  return Object.keys(require.cache || {});
+};
+
+/**
+ * Flush test files from require.cache
+ *
+ * @api private
+ */
+Mocha.prototype._flushRequireCache = function() {
+  this._getRequireCache().forEach(function(filePath) {
+    if (this._requireCache.indexOf(filePath) === -1) {
+      delete require.cache[filePath];
+    }
+  }, this);
+};
+
 /**
  * Escape string and add it to grep as a regexp.
  *

test/jsapi/cache/include.js

@@ -0,0 +1,4 @@
+module.exports = function() {
+  global[__filename] = global[__filename] || 0;
+  global[__filename]++;
+};

test/jsapi/cache/index.js

@@ -0,0 +1,20 @@
+var Mocha = require('../../../')
+  , assert = require('assert')
+  , path = require('path');
+
+var testFile = path.resolve(__dirname, 'spec.js');
+var includeFile = path.resolve(__dirname, 'include.js');
+
+var set1 = new Mocha();
+set1.addFile(testFile);
+
+var set2 = new Mocha();
+set2.addFile(testFile);
+
+set1.run(function () {
+  assert.strictEqual(global[includeFile], 1, 'JSAPI test #1 didn\'t run');
+
+  set2.run(function () {
+    assert.strictEqual(global[includeFile], 2, 'JSAPI test #2 didn\'t run');
+  });
+});

test/jsapi/cache/spec.js

@@ -0,0 +1,8 @@
+var path = require('path');
+var increment = require('./include');
+
+describe('Mocha', function () {
+  it('should increment global counter', function () {
+    increment();
+  });
+});