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...