Add 'test' target to unix/Makefile

In conjunction with #504 this allows you to do things like:
```shell
make -C unix clean && make -C unix test CC=gcc-4.7 PROG=micropython-gcc4.7
```
all from the top-level micropython directory :-)

Something similar could probably be done for windows/Makefile too, but I don't have a cygwin setup to test with.