@isTest
public class permissionSetTest {

    @isTest 
    static void validatePermissionSetTest()
    {
        Test.startTest();
            
        PermissionSet.addPermissionSet();
        
		List<PermissionSetAssignment> psa = new List<PermissionSetAssignment>([select id, assignee.name, assignee.userrole.name, assignee.profile.name, assignee.isactive from permissionsetassignment where assignee.profile.name in ('Integration Administrator') and assignee.isactive = true and permissionset.name = 'testps1']); 
        
        system.AssertEquals(True, psa.size() == 2);
        
        Test.stopTest();
    }
}

public class permissionSet {
    public static void addPermissionSet() {
        // Create a list of permission list assignments.
        List<permissionSetAssignment> permissionSetList = new List <PermissionSetAssignment> ();
        
        // Get the permission set id by name.
        Id permissionSetId = [Select id from permissionset where name = 'testps1'].id;
        
        // Create a string variable to use to find users to assign to the permission set.
        Set<String> profileSet = new Set<String>{'Integration Administrator'};
        
        // Get users by the profile name (2 users in this case).
        for (User u : [select id, userrole.name, profile.name, isactive from user where profile.name in :profileset and isactive = true])
	    {
          	// Create the new permission set assignment instance.
          	PermissionSetAssignment psa = new permissionsetassignment(PermissionSetId = permissionSetId, AssigneeId = u.id);
            // Add the new permission set assignment to the psa list.
            permissionSetList.add(psa);
    	}
        // Apply the psa.
        upsert permissionSetList;
    }
}