test: improve Sigdump test for Windows (#4847)

**Which issue(s) this PR fixes**: 
None.
Related to

* #4063
* #4846

**What this PR does / why we need it**: 
Sigdump.dump iterates ObjectSpace.
If there are DoubleDefinition objects of RR, it can set some
expectations of RR unexpectedly.
It can cause test failure.

By doing GC, we can remove DoubleDefinition objects made in other tests.
This allows us to simplify this test.

See #4063

**Docs Changes**:
Not needed.

**Release Note**: 
Not needed.

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

Author: daipom

test/test_supervisor.rb

@@ -490,34 +490,31 @@ def test_fluentsigdump_get_path_with_pid(data)
   def test_supervisor_event_dump_windows
     omit "Only for Windows, alternative to UNIX signals" unless Fluent.windows?
 
+    # https://github.com/fluent/fluentd/issues/4063
+    GC.start
+
+    ENV['SIGDUMP_PATH'] = TMP_DIR + "/sigdump.log"
+
     server = DummyServer.new
     def server.config
       {:signame => "TestFluentdEvent"}
     end
     server.install_windows_event_handler
 
-    assert_rr do
-      # Have to use mock because `Sigdump.dump` seems to be somehow incompatible with RR.
-      # The `mock(server).restart(true) { nil }` line in `test_rpc_server_windows` cause the next error.
-      # Failure: test_supervisor_event_dump_windows(SupervisorTest):
-      #   class()
-      #   Called 0 times.
-      #   Expected 1 times.
-      # .../Ruby26-x64/lib/ruby/gems/2.6.0/gems/sigdump-0.2.4/lib/sigdump.rb:74:in `block in dump_object_count'
-      #     73: ObjectSpace.each_object {|o|
-      #     74:   c = o.class <-- HERE!
-      mock(Sigdump).dump(anything)
-
-      begin
-        sleep 0.1 # Wait for starting windows event thread
-        event = Win32::Event.open("TestFluentdEvent_CONT")
-        event.set
-        event.close
-        sleep 1.0 # Wait for dumping
-      ensure
-        server.stop_windows_event_thread
-      end
+    begin
+      sleep 0.1 # Wait for starting windows event thread
+      event = Win32::Event.open("TestFluentdEvent_CONT")
+      event.set
+      event.close
+      sleep 1.0 # Wait for dumping
+    ensure
+      server.stop_windows_event_thread
     end
+
+    result_filepaths = Dir.glob("#{TMP_DIR}/*")
+    assert {result_filepaths.length > 0}
+  ensure
+    ENV.delete('SIGDUMP_PATH')
   end
 
   data(:ipv4 => ["0.0.0.0", "127.0.0.1", false],