Backport(v1.19): test_buffer: revert changes to avoid failure (#5396) (#5397)

**Which issue(s) this PR fixes**: 
Backport #5396
Fixes #

**What this PR does / why we need it**: 
The test changes introduced appear to be causing CI failures.

6c88e14

**Docs Changes**:

**Release Note**:

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Shizuo Fujita <fujita@clear-code.com>

Author: github-actions[bot]

test/plugin/test_buffer.rb

@@ -1454,8 +1454,8 @@ def create_chunk_es(metadata, es)
       @p = create_buffer({'compress' => 'gzip', 'chunk_limit_size' => 70})
       timestamp = event_time('2016-04-11 16:00:02 +0000')
       es = Fluent::ArrayEventStream.new([[timestamp, {"message" => "012345"}], # overflow
-                                         [timestamp, {"message" => "a"}],
-                                         [timestamp, {"message" => "b"}]])
+                                         [timestamp, {"message" => "aaa"}],
+                                         [timestamp, {"message" => "bbb"}]])
       assert_equal [], @p.queue.map(&:metadata)
       assert_equal 70, @p.chunk_limit_size