During the tests, many attachment can be created and unlink.
They will be eventually cleaned up if the cron is ran but this is
usually not the case during tests. The disk usage can increase to reach
more than one 1 Go when design theme is installed.
This can lead to unnecessary big database dumps.
This is also a problem on odoosh where the filestore max size is limited
to 1Go.
The gc should be quite fast if nothing was changed since it will just
check the content of an empty directory.
The method is made accessible in the test case in order to be able to gc
on demand. This may be useful in the test_01_crawl_every_themes that
can generate around 600~ Mo of attachment in the loop.
X-original-commit: 187309f5a39f8fef9b07959fd73475fd6732efb3
Part-of: odoo/odoo#102502
Xavier-Do
4e9ad91107