Drop test/fixtures from npm bundle

Notice the published npm library contains all the `test/fixtures` which is about 6.7MB (95% of the size of the library).

https://unpkg.com/martinez-polygon-clipping@0.1.5/test/fixtures/

```
$ du -h
4.0K	./demo/css
260K	./demo/js
272K	./demo
 64K	./dist
 64K	./src
6.7M	./test/fixtures
6.7M	./test
7.2M	.
```