How to make Pipeline job to wait for all triggered parallel jobs?

You should use pipeline parallel expression, which will wait for all spawned jobs / subtasks to complete:

stage('testing') {
    def branches = [:]

    for(i = 0; i < params.size(); i += 1) {
        def param = params[i]

        branches["Test${i}"] = {
            build job: 'Test', parameters: [string(name: 'Name', value: param)], quietPeriod: 2
        }
    }
    parallel branches
}

You can find some more examples in pipeline docs at jenkins.io

Just run into the same problem, and find a working solution. Just use foreach.

stage('testing') {
    def jobs = [:]

    [1,2,3,4,5].each{
        i -> jobs["Test${i}"] = {
            build job: 'Test', 
            parameters: [string(name: 'theparam', value: "${i}")],
            quietPeriod: 2
        }
    }
    parallel jobs
}