cmd/llgo-build: Add -test flag creating a test binary. For #48.

This is able to create a binary for some packages, although I'm
having issues actually running them. Will comment in #48 with more
details.

This PR depends on having merged the others first.
cmd/llgo-build: add missing test.go, flag.Args() -> args
Merge pull request #90 from quarnster/test

cmd/llgo-build: Add -test flag creating a test binary. For #48.