Fix PostgreSQL range tests

The changes in c4044b2 meant the tests would error on a fresh DB.
Unused classes in AMo tests
Merge pull request #13393 from matthewd/fix_pg_range_tests

Fix PostgreSQL range tests on fresh DB

Correcting the name of the table we're creating is self-explanatory.

But we must also move away from the low IDs, because we're not touching the freshly-created primary key sequence; when the time comes, @new_range will be assigned an ID of 1.
Update configuring guide with I18n options regarding available locales

Add information about 'available_locales' and 'enforce_available_locales'
to the configuring guide.

Closes #13202. [ci skip]
Fix asserting the correct exception message in dependencies test

In Minitest, the second argument of assert_raise(s) accepts a string as
the message that should be shown in case of a failure in the assertion
(eg nothing was raised when it should), and not the exception message to
be matched.

To do that we need to save the exception returned from assert_raise(s)
into a local variable and check for the exception message using it.
Prefer assert_raise instead of flunk + rescue to test for exceptions

Change most tests to make use of assert_raise returning the raised
exception rather than relying on a combination of flunk + rescue to
check for exception types/messages.
Show routes defined under assets prefix

Closes #9625