We should only display commands in detail strings, not in the
message texts.

This also updates tests/integration/sandbox-bwrap.py to expect
the new message string which only contains the command exit status
and not the whole command itself, this does not alter the validity
of the text case which is checking that we can obtain the expected
return value.

General refactor in tests directory

See merge request BuildStream/buildstream!1062

This is the directory for all things related to loading.

The remaining test simply loads a project with one element
and asserts a value on it. This is already sufficiently tested
in tests/format/project.py.

Remove some redundancy from the test lines.

This is where other load time related plugin error handling is
checked, and is the last thing to remove in the `tests/pipeline`
directory.

The tests/format/project.py test already has some tests about how
we error gracefully for bad plugins and bad plugin configurations,
lets put it there rather than tests/pipeline/load.py which we
will remove.

Created new `tests/format/iteration.py` which tests the order in
which elements are iterated over in various scopes in a loaded
data model.

This used to be an internal test, converted this to use the `cli` fixture.