Can't stop server running by nodemon

I create gulp task for unit tests. I add nodemon for automatically run server then run test. But have error when run gulp task again. I have error that the port is already busy with another process.

I user this code:

var gulp = require('gulp'),
    gulpUtil = require('gulp-util'),
    gulpShell = require('gulp-shell'),
    gulpEnv = require('gulp-env'),
    gulpNodemon = require('gulp-nodemon'),
    gulpMocha = require('gulp-mocha');

gulp.task('default', function () {
    gulpUtil.log('unit - run unit tests');
});

gulp.task('server', function (callback) {
    var started = false;

    return gulpNodemon({
        script: './build/app.js'
    })
        .on('start', function () {
            if (!started) {
                started = true;

                return callback();
            }
        })
});

gulp.task('unit', ['server'], function () {
    return gulp.src('./src/*.js')
        .pipe(gulpMocha({reporter: 'spec'}))
        .once('error', function () {
            process.exit(1);
        })
        .once('end', function () {
            process.exit();
        })
});

How I can stop or kill server after unit tests?

via Metal Evolution Studio