[LINTER] Fix the tests linter

Summary:
The test linter only works on test suites that are using the
BOOST_FIXTURE_TEST_SUITE macro, however some tests don't require any
fixture and use BOOST_AUTO_TEST_SUITE.

This diff also adds the new seeder test directory to the linter paths.

Depends on D4417.

Test Plan:
Show no error:

arc lint --everything

Show both mismatch and duplicated name errors:
Copy the seeder test file with the name seeder2_tests.cpp then run:

arc lint

Reviewers: #bitcoin_abc, deadalnix, nakihito, jasonbcox

Reviewed By: #bitcoin_abc, nakihito, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4718