excluded_terms filter in `get_adjacent_link()` doesn't do anything

[boonebgorges]

The get_{$adjacent}_post_excluded_terms filter, added in [34528], doesn't do anything. It runs after the excluded terms have been added to the SQL clause.

Solution is to move it further up in the function.

See #9571.

[boonebgorges]

In 36077:

Move get_adjacent_post() tests to their own file.

See #35211.

[boonebgorges]

In 36078:

Move excluded_terms filter in get_adjacent_post().

The filter was added in 4.4 [34528] #9571, but in a place where it could not
affect the adjacent post query.

Fixes #35211.

[netweb]

The @covers annotation added in [36077] references a non-existent function get_adjacent_link(), it should reference get_adjacent_post() as such 35211.diff​ changes this to @covers ::get_adjacent_post

[boonebgorges]

In 36159:

Fix incorrect @covers annotation in get_adjacent_post() tests.

Introduced in [36077].

Props netweb.
Fixes #35211.