fix(workers): distribute only tests matching grep

[DavertMik]

Problem

run-workers splits all loaded tests across workers and each worker applies --grep afterwards. With a narrow pattern the matching tests can land on one worker, while the others start only to print "No tests found by pattern".

On the sandbox fixture (7 tests, 2 match):

$ codeceptjs run-workers 3 --grep "From worker"
Test distribution:
  Worker 1: 3 tests
  Worker 2: 2 tests
  Worker 3: 2 tests

Change

• createGroupsOfTests / createGroupsOfSuites assign only tests matching mocha's grep. That covers --grep, --invert, the grep config key and Feature.only / Scenario.only.
• Workers that would get no tests are not started (at least one is always kept).
• When nothing matches, distribution is unchanged, so workers still print the empty-run hint and CI still fails on an empty run.

After:

$ codeceptjs run-workers 3 --grep "From worker"
Test distribution:
  Worker 1: 1 test
  Worker 2: 1 test

Not changed

• --by pool hands out files, not tests, so it still loads files that have no matching test.
• The "Running tests in N workers..." banner still prints the requested count.
• Groups returned by createGroupsOfTests(n) to custom scripts keep length n; only the built-in split drops empty ones.

Tests

test/runner/run_workers_test.js and test/unit/worker_test.js pass locally (45 passing, 2 pending).

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