test_supervisor: use expected temporary directory for tests (#5102)

**Which issue(s) this PR fixes**:
Fixes #

**What this PR does / why we need it**:
test_supervisor.rb uses a constant named `TMP_DIR`.
However, the constant is not defined in this file, it is defined in
another file.

https://github.com/fluent/fluentd/blob/5a875090dde0222edf789422c7b2f27a828c72c3/test/config/test_dsl.rb#L5

This PR will use expected temporary directory for tests.

**Docs Changes**:
N/A

**Release Note**:
N/A

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

test/test_supervisor.rb

@@ -405,7 +405,7 @@ def test_supervisor_event_dump_windows
     # https://github.com/fluent/fluentd/issues/4063
     GC.start
 
-    ENV['SIGDUMP_PATH'] = TMP_DIR + "/sigdump.log"
+    ENV['SIGDUMP_PATH'] = @tmp_dir + "/sigdump.log"
 
     server = DummyServer.new
     def server.config
@@ -423,7 +423,7 @@ def server.config
       server.stop_windows_event_thread
     end
 
-    result_filepaths = Dir.glob("#{TMP_DIR}/*")
+    result_filepaths = Dir.glob("#{@tmp_dir}/*")
     assert {result_filepaths.length > 0}
   ensure
     ENV.delete('SIGDUMP_PATH')