make option "testFileExtensions" accept an array like ['js','coffee']

If set "testFileExtensions": [ 'js', 'coffee']
in original code: this._nodeHasteTestRegExp is //__tests__/.*\.(js\|coffee)$/

This make Jest can't find tests: 0 Found 3 matching tests...

The change calls escapeStrForRegex() before join the string, so the gengerated regexp is:
//__tests__/.*\.(js|coffee)$/

Now Jest can find tests:
Found 3 matching tests...