Adds a variant-type filter

Also correctly calculates true/false/pos/neg etc for SVs.
You could also do it this way, up to you. Most test frameworks will give very good error messages.
```javascript
assert.equal(trueFalsePositiveNegativeForSVs(svs, svsTruth), {
    svTruePositives: 1,
    svFalsePositives: 2,
    svFalseNegatives: 1
});
```