test: refactor code (#5013)

**Which issue(s) this PR fixes**: 
* For #5012

**What this PR does / why we need it**: 
Refactor code for #5012

**Docs Changes**:
Not needed.

**Release Note**: 
Not needed because this is new code for v1.19.

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

Author: daipom

test/plugin/test_buf_file.rb

@@ -23,7 +23,7 @@ def write(chunk)
 
   class DummyErrorOutputPlugin < DummyOutputPlugin
     def register_write(&block)
-      instance_variable_set("@write", block)
+      instance_variable_set(:@write, block)
     end
 
     def initialize

test/plugin/test_buf_file_single.rb

@@ -40,7 +40,7 @@ def write(chunk)
 
   class DummyErrorOutputPlugin < DummyOutputPlugin
     def register_write(&block)
-      instance_variable_set("@write", block)
+      instance_variable_set(:@write, block)
     end
 
     def initialize