Confirm to have tests equivalent with embulk-core

Java `src/test/java`

* [ ] `org/embulk/config/TestConfigSource.java`
* Including some tests w/ `TimestampParser`, but just using.
* [x] `org/embulk/spi/time/TestRubyTimeFormat.java`
* Same tests in `org/embulk/util/rubytime/TestFormat.java`
* [X] `org/embulk/spi/time/TestTimeZoneIds.java`
* Most regional timezones (ex. Asia/Taipei) are not supported in `embulk-util-rubytime`.
* Some are tested in `org/embulk/util/rubytime/TestTimeZones.java`.
* To be tested with `LegacyEmbulkZones.java` on the Embulk side.
* [x] `org/embulk/spi/time/TestTimestamp.java`
* `Timestamp` is Embulk's original class.
* Not tested in `embulk-util-rubytime`.
* [x] `org/embulk/spi/time/TestTimestampFormatter.java`
* Formatter is not implemented yet.
* Not tested in `embulk-util-rubytime`.
* [x] `org/embulk/spi/time/TestTimestampFormatterParser.java`
* It consists of tests as `org.embulk.spi.Timestamp`.
* Not tested in `embulk-util-rubytime`.
* [x] `org/embulk/spi/time/TestTimestampParser.java`
* These tests are to be imported.
* [x] `org/embulk/spi/time/TestTimestampSerDe.java`
* These tests are just for SerDe.
* Not tested in `embulk-util-rubytime`.

Ruby `src/test/ruby`

* [ ] `monkey_strptime/mri/date/test_date_strptime.rb`
* Tested with `org/embulk/util/rubytime/TestRubyDateTimeFormatterParseWithJRuby.java` and `src/test/resources/ruby/test/date/test_date_strptime.rb`
* [x] `vanilla/test_schema.rb`
* Not to be tested in `embulk-util-rubytime`.
* [x] `vanilla/guess/test_schema_guess.rb`
* Not to be tested in `embulk-util-rubytime`.
* [x] `vanilla/guess/test_time_format_guess.rb`
* Not to be tested in `embulk-util-rubytime`.
* [ ] `vanilla/time/test_timestamp_parser.rb`
* Some are identical with `test_date_strptime.rb`, but need to be investigated.