Call `test.img` - not `test` - target and express deps

See last line below:
```
$ git grep '\.\./' | grep '^\([A-z]\+\/\)\+[A-z]\+:'
...
test/Makefile:4:        ../railing/railing image -ltest_server -lcompiler -lcrypto -lsyntax_tools -lsasl -los_mon -ipriv -m 1024
```