This was previously append_required_artifacts(), which presumed that
we knew at startup time what the cache keys of all elements in the
loaded pipeline would be.

This fixes unexpected deletions of required artifacts when
dynamic tracking is enabled with `bst build --track-all target.bst`

This commit renames test_never_delete_dependencies() to
test_never_delete_required(), renders the test more readable by renaming
some elements and reordering some statements and makes the comments more
straight forward and accurate.

These tests were not checking that we fail for the expected reasons.

Added `res.assert_task_error(ErrorDomain.ARTIFACT, 'cache-too-full')`
where we expect to fail because the cache is too full.

This allows one to modify a file in an existing git repo,
as opposed to adding a new one.

 * create_element_size()

   Now uses a git Repo object instead of a local source, and
   returns the repo.

 * update_element_size()

   Added this function which can now resize the expected output
   of an element generated with create_element_size(), useful
   to allow testing sized elements with the tracking feature.

Same test as test_never_delete_required(), except that this test ensures
that we never delete required artifacts when their cache keys are
discovered dynamically during the build.

Don't delete required artifacts when tracking is enabled

See merge request !793