This commit adds helpers and improves on the way we assert XML files in `AccountTestInvoicingCommon` and all accounting test that extend from it. From now on, all accounting test code that assert an XML tree/string to an XML file should call the `assert_xml` helper, and design their test file name/location/etc. around this framework.

This approach has a few major benefits:

### Assert / Save XML

When testing XML files, we often need to perform create/read/update operations on the asserted XML to make sure it corresponds to the most updated/intended data. Previously, to save something to an XML, a developer would need to write their own local helpers to save the XML in the right directory. This was cumbersome and error-prone, so we decided to design a helper that allows developer to immediately save AND/OR update the asserted XML: to save/update an XML, we can simply add `SAVE_XML` as an additional test tags.

### Better test naming and optional subfolder management

To better organize test files, the `assert_xml` method allows us to write just the test key name (without `.xml`), and the framework will automatically get the XML to assert/save from the `test_files` directory. An optional `subfolder` parameter is also added to allow writing to specific subfolder within `test_files`.

### Better `___ignore___` management in assertion XMLs

Sometimes, we want to ignore a few XML node that are not relevant, or have content that are not deterministic (changes on every test run). To handle this, previously, developers would need to modify the assertion XML content by hand or write their own local script to do so.

With this new framework, we just need to add an `ignore_schema.xml` file somewhere in the `test_files` directory. If put inside a subfolder, it will be applied with more priority towards the XML that are put on that specific subfolder.

### Save "pure" XML (before applying `___ignore___`) in temporary folder

When calling `SAVE_XML`, before applying the ignore patches, the XML will be saved in a temporary folder (same folder as the screenshots for tours), so that developers can use them in external tests in the future, and for any other saving reasons.

In addition, this commit also:

- add `edi_tags` helper to save all the common tags for EDIs, which also enables `EXTERNAL_MODE` testing inspired by `l10n_mx_edi`
- convert some non-assert XML test helpers into a class method
- add `XmlCollector` exception to be used to stop a method and collect an XML data for tests in EDIs. (see `l10n_gt_edi`)
- refactor `l10n_mx_edi*` modules to use these helpers

related-community-PR: https://github.com/odoo/odoo/pull/235565

task-4891206