Debugging/improvement of an at install test may be tedious because of
the need to update the module, spending most of the time checking
tables and xml file.

1. This commit proposes to allow to execute test without installing or
updating a module. The test is still executed during the loading, and
the behavior should be close to an execution of tests during an update.

Co-authored-by rco-odoo <rco@odoo.com>

2. keep previous behavior if -i or -u is given

Three solutions were possible:
- The clean one that changes dev habits.
When test-enable or test_tags is given, all tests are always executed,
a test_tags is needed to select tests to execute:
Example: `-u module --test_enable` becomes `-u module --test-tags /module` to keep the same behaviour
This solution is the simplest, and executed tests does not depends on database already installed modules.
- The conservative solution.
When giving -i or -u, the behavior stays the same as before. When giving test-enable
without -i/-u, test are executed on all installed modules.
- The intermediate solution:
When no test_tags is given but a -i and -u is given, only the given modules are tested.
This is quite close to the second solution except that a -i module on a new database won't
test all dependencies on the first install.

The chosen solution is the second one to minimize changes on dev old habits,
only an almost unused feature is impacted: using test-enable without any -i or -u.
Before this pr only post install tests were executed in this case. Now at_install tests are also executed.
This combination is actually used by runbot to execute post_install test in parallel, but a `--test-tags -at_install`
tag is given so nothing to worry about here.

closes odoo/odoo#53499

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>