[CLN] tests: default test tag is post_install

[fw-bot]

Since saas-19.1*, default tag for tests is post_install, where it was at_install before.
This commit removes the useless post_install redefinition of the tag.

*odoo/odoo@18a7258

Forward-Port-Of: #1988

[robodoo]

[fw-bot]

@vava-odoo cherrypicking of pull request #1988 failed.

stdout:

Auto-merging tests/test_beverage_distributor/tests/test_action_server.py
Auto-merging tests/test_booking_engine/tests/test_action_server.py
Auto-merging tests/test_booking_engine/tests/test_computed_fields.py
Auto-merging tests/test_construction_developer/tests/test_action_server.py
Auto-merging tests/test_generic/tests/test_industry_mail_behavior.py
CONFLICT (content): Merge conflict in tests/test_generic/tests/test_industry_mail_behavior.py
Auto-merging tests/test_generic/tests/test_xml.py
Auto-merging tests/test_hvac_services/tests/test_action_server.py
Auto-merging tests/test_hvac_services/tests/test_computed_fields.py
Auto-merging tests/test_shoe_maker/tests/test_action_server.py

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

you can use git-fw to re-do the forward-port for you locally.

⚠️ after resolving this conflict, you will need to merge it via @robodoo.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[vava-odoo]

@robodoo r+

[robodoo]

@vava-odoo 'ci/industry' failed on this reviewed PR.

[vava-odoo]

@robodoo override=ci/industry
@robodoo r+

[robodoo]

This PR is already reviewed, reviewing it again is useless.