Show sniff code in error report about failed unit test

During sniff development it's important to easily determine what caused test to fail. Unfortunately it's not always possible to do just by reading error message text. Here I've added a sniff code to the error report as well.

Before:
```
[LINE 155] Expected 0 error(s) in FunctionCommentUnitTest.inc but found 2 error(s). The error(s) found were:
 -> Doc comment for var &$str does not match actual variable name $str at position 1
 -> Param comment must end with a full stop
```

After:
```
[LINE 155] Expected 0 error(s) in FunctionCommentUnitTest.inc but found 2 error(s). The error(s) found were:
 -> Doc comment for var &$str does not match actual variable name $str at position 1 (Squiz.Commenting.FunctionComment.ParamNameNoMatch)
 -> Param comment must end with a full stop (Squiz.Commenting.FunctionComment.ParamCommentFullStop)
```