test_filter_record_transformer: pass a Time object to Timecop.freeze (fluent#5077)

**Which issue(s) this PR fixes**: 
Fixes fluent#5041

**What this PR does / why we need it**: 
I'm investigating fluent#5041

When run the `RecordTransformerFilterTest`,
it handles incorrect timestamps.

Reproduce:

```ruby
    @time = event_time('2010-05-04 03:02:01 UTC')
    Timecop.freeze(@time)

    logger = Logger.new(STDOUT)
    p logger.info("Test::RecordTransformerFilterTest setup")
```

Result:
```
I, [1272942121-01-01T00:00:00.000000 #12576]  INFO -- : Test::RecordTransformerFilterTest setup
```

We should pass Time object into `Timecop.freeze`


**Docs Changes**:
Not needed

**Release Note**: 
CI fixes

---------

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

Author: Watson1978

test/plugin/test_filter_record_transformer.rb

@@ -14,7 +14,7 @@ class RecordTransformerFilterTest < Test::Unit::TestCase
     @tag = 'test.tag'
     @tag_parts = @tag.split('.')
     @time = event_time('2010-05-04 03:02:01 UTC')
-    Timecop.freeze(@time)
+    Timecop.freeze(@time.to_time)
   end
 
   teardown do