The tests/pipeline directory will be removed, and this test
is the better of the two but also redundant with the one in
tests/frontend/show.

Renamed existing test in show.py to `test_show_except_simple`
and added the better test as `test_show_except` below it.

Grouping bits of internal testing together here

This tests exactly the same thing that is tested in
tests/internals/pluginfactory.py (the new location
of tests/plugins/basics.py).

Now that the remaining test "filter.py" in the plugins directory
tests a specific element, it makes sense to create a place for
testing elements, just like we do for sources.

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

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

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.

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

Remove some redundancy from the test lines.

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.

This is the directory for all things related to loading.

General refactor in tests directory

See merge request !1062

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.