public RunResult runOutput(ConfigSource outConfig, Path inputPath, ConfigSource execConfig)
            throws IOException {
        return outputBuilder()
                .exec(execConfig)
                .out(outConfig)
                .inputPath(inputPath)
                .run();
    }
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