testsuite: Use '|| true' when pipefail is involved ...

On Mac OSX, this script fails because of the behavior of pipefail.
A simple ' || true' solves the problem, while 'grep' still ensures
it behaves as expected.

add non-interactive mode to build command ...

- useful to build already fetched package
- skips online search and interactive fetch dialog

dub run <package> should search the registry on failure