Remove eunit from default target

I'm using [erlang.mk](https://github.com/ninenines/erlang.mk) and having `eunit` as part of the default target is kind of annoying, because every time I run my tests the `triq` EUnit tests are run first.
This is because erlang.mk simply calls `make -C deps/triq`, which will call the default target.

GNU defines some ["standard" targets](https://www.gnu.org/prep/standards/html_node/Standard-Targets.html), in which they say that `all` should basically just compile the entire program.

This PR removes the `eunit` switch from the default target and adds two new ones:

`eunit`: To run the EUnit tests
`test`: Run the `eunit` and `qc` targets