Use omit instead of return in assertion

Author: hsbt

tool/lib/core_assertions.rb

@@ -390,7 +390,7 @@ def assert_separately(args, file = nil, line = nil, src, ignore_stderr: nil, **o
 
       # Run Ractor-related test without influencing the main test suite
       def assert_ractor(src, args: [], require: nil, require_relative: nil, file: nil, line: nil, ignore_stderr: nil, **opt)
-        return unless defined?(Ractor)
+        omit unless defined?(Ractor)
         omit if windows?
 
         # https://bugs.ruby-lang.org/issues/21262