`--pattern` ignores list of test files

[locker]

Running all tests defined in a test file:

$ ./test-run.py --builddir ../build/debug box-luatest/app_threads_test.lua
<...>
================================================================================
WORKR TEST                                            PARAMS        RESULT
--------------------------------------------------------------------------------
[001] box-luatest/app_threads_test.lua                              [ pass ]
--------------------------------------------------------------------------------

Now trying to run only fiber-related tests defined in this file:

./test-run.py --builddir ../build/debug --pattern fiber box-luatest/app_threads_test.lua
<...>
================================================================================
WORKR TEST                                            PARAMS        RESULT
--------------------------------------------------------------------------------
[004] unit/iterator_position.test                                   [ pass ]
[004] unit/raft.test                                                [ pass ]
[004] unit/serializer.test                                          [ pass ]
[004] unit/column_mask.test                                         [ pass ]
[004] unit/uri.test                                                 [ pass ]
[004] unit/tt_static.test                                           [ pass ]
[004] unit/event.test                                               [ pass ]
[004] unit/vy_cache.test                                            [ pass ]
[004] unit/ratelimit.test                                           [ pass ]
[015] app-luatest/console_debugger_session_test.lua                 [ skip ]
[007] config-luatest/iproto_ssl_test.lua                            [ skip ]
[009] vinyl-luatest/gh_10512_page_load_error_test.lua               [ skip ]
[004] unit/xlog.test                                                [ pass ]
[017] sql-luatest/gh_10243_varbinary_bound_variable_>               [ skip ]
[004] unit/error.test                                               [ pass ]
[021] replication-luatest/gh_8757_bootstrap_crash_te>               [ skip ]
[004] unit/fiber.test                                               [ pass ]

WTF?!