test: ensure removing old engine on setup (fluent#5057)

**Which issue(s) this PR fixes**:
Continued from

* fluent#5054
* fluent#5055 (6cac9f0)

**What this PR does / why we need it**:
Each test should not consider the initialization of `Fluent::Engine`.
It should be the responsibility of `Fluent::Test.setup`.

Note: Set `nil` explicitly to ensure that GC can remove the objects,
though it is very strange that some objects can still exist after
`remove_const` and `GC.start`.

**Docs Changes**:
Not needed.

**Release Note**:
CI improvements.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

lib/fluent/test.rb

@@ -40,9 +40,12 @@ def self.setup
 
       $log = dummy_logger
 
-      Fluent.__send__(:remove_const, :Engine)
-      engine = Fluent.const_set(:Engine, EngineClass.new).init(SystemConfig.new)
+      old_engine = Fluent.__send__(:remove_const, :Engine)
+      # Ensure that GC can remove the objects of the old engine.
+      # Some objects can still exist after `remove_const`. See https://github.com/fluent/fluentd/issues/5054.
+      old_engine.instance_variable_set(:@root_agent, nil)
 
+      engine = Fluent.const_set(:Engine, EngineClass.new).init(SystemConfig.new)
       engine.define_singleton_method(:now=) {|n|
         @now = n
       }