buildroot: return a new CompletedBuild with output

Create a new CompletedBuild object that wraps and is very similar
to the subprocess.CompletedProcess, i.e. it has a process member
but also has shortcuts for returncode. Additionally, the output
of the process is not only forwarded to the monitor, but also
captured and then handed to CompletedBuild, so its output member
will actually contain the full build output. To be compatible
with the previously returned CompletedProcess, `stderr`, `stdout`
members exist on CompletedBuild that also return `output`.

test/mod/test_buildroot.py

@@ -68,6 +68,8 @@ class TestBuildRoot(test.TestBase):
         with open(logfile) as f:
             log = f.read()
         assert log
+        assert r.output
+        self.assertEqual(log, r.output)
 
     @unittest.skipUnless(test.TestBase.have_test_data(), "no test-data access")
     def test_bind_mounts(self):