`luarocks test`: build before running the test suite

## Problem

Compiled modules (e.g. written in C or built with a custom build backend) cannot be `require`d in test suites, unless one builds them first.

This leads to an unergonomic sequence:

Before running tests, I have to

1. Build the project

```bash
mkdir .luarocks
luarocks $LUAROCKS_EXTRA_ARGS make --tree=.luarocks --deps-mode=all
```

2. Add the build directory to the `package.cpath` (e. g. in a `.busted` file):

```
cpath = "./.luarocks/lib/lua/5.1/?.so",
```

3. Run `luarocks test`

## Proposed solution

It would be great if luarocks had a mechanism to build the project and set up the Lua paths (perhaps including dependencies) before running the test suite.

### Pros/cons

(+) Simpler UX
(+) This is what other package managers do (cargo, cabal, stack, gradle, ...)
(-) Extra overhead of potentially building the package more than once

The extra overhead could potentially be mitigated by caching a checksum of the source directories (and `copy_directries`?) and only rebuilding if it has changed.

Start at the `luarocks test` entry point and compare it with the provided `luarocks ... make --tree=.luarocks --deps-mode=all` flow; inspect how `.busted` supplies `package.cpath`. Done means test runs can build the project and configure Lua paths, including dependencies, without those manual steps.

Written by the indexing model from the issue text.