[FIX] base: attachment copy ACL

Currently, `Attachment.copy` has an *explicit* check for write access
on the underlying record.

This doesn't necessarily make sense e.g. a user copying an attachment
from a template to an email may not have write access to the template,
but that should not be an issue. And indeed if the copy is performed
"by hand" (read then create) things work fine[^1].

Since both `read` and `create` are checked, the explicit `copy` check
doesn't seem necessary. Drop it, and try to add some tests around
`copy`. Move `test_06_linked_record_permission` to its own `TestCase`
and split it for readability.

A secondary issue is that the `check` in `create` was performed in the
context of the `self` being copied, leading to the same issue as
`copy` being repeated (that is, `a.copy()` would call `a.create()`
which would call `a.check('write')`, even though we're semantically
creating a record from scratch). The behavior is really intended for
`write` where we want to check if we have `write` access to both the
"source" and the "destination" records.

Explicitly opt-out of having any source data in the `create` before
performing the `check` calls, this way we correctly and only check for
the writability of the destination, and only check for the readability
of the source.

issue 2746483

[^1] well not entirely true, see 4th paragraph

closes odoo/odoo#84580

X-original-commit: bb956d7d
Signed-off-by: Raphael Collet <rco@odoo.com>
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>