public ConfigDiff guessParser(ConfigSource parserSeedConfig, Path inputPath, ConfigSource execConfig) {
        return parserBuilder()
                .parser(parserSeedConfig)
                .inputPath(inputPath)
                .exec(execConfig)
                .guess();
    }
This method returns PreviewResult.

@return PreviewResult returns the result by PreviewExecutor
@throws IOException in case of failures in file operations
embulk-junit4/src/main/java/org/embulk/test/TestingEmbulk.java