[bldr-build] Add an optional `do_check()` phase for post-compile tests.
[bldr-build] Add an optional `do_check()` phase for post-compile tests.

This new phase will run post-compile tests and checks, provided 2
conditions are true:

1. A `do_check()` function has been declared. By default, no such
   function exists, so Plan author must add one explicitly--there is no
   reasonably good deault here.
1. A `$DO_CHECK` environment variable is set to some non-empty value. As
   tests can dramatically inflate the build time of a Plan, this has been
   left as an opt-in option.

Here's an example example of a vanilla Plan such as Sed:

```sh
pkg_name=sed
 # other Plan metadata...

do_check() {
  make check
}
```

be-plans/be,be-plans/be,be-plans/be,be-plans/be