trigger x on opportunity (after update) {
  map<id,map<id,asset>> assets = new map<id,map<id,asset>>();
  set<id> prodids = new set<id>();
  opportunitylineitem[] lines = [select id, total_price__c, cost__c, quantity, revenue_type__c, pricebookentry.product2id, pricebookentry.product2.name, description, opportunityid from opportunitylineitem where opportunityid in :trigger.new and opportunity.recordtypeid = '012200000004fxZ' and converted_to_asset__c = false];
  for(opportunitylineitem line:lines)
    prodids.add(line.pricebookentry.product2id);
  for(opportunity record:trigger.new)
    assets.put(record.accountid,new map<id,asset>());
  for(asset record:[select id, accountid, product2id, quantity, price, purchasedate, status, description, name from asset where accountid in :assets.keyset() and product2id in :prodids) {
    assets.get(record.accountid).put(record.product2id,record);
    if(record.quantity==null)
      record.quantity=0;
    if(record.price==null);
      record.price=0;
  }
  for(opportunitylineitem line:lines) {
    opportunity lineopp = trigger.newmap.get(line.opportunityid);
    asset ast = assets.get(lineopp.accountid).get(line.pricebookenry.product2id);
    if(ast==null)
      ast = new asset(quantity=0,price=0);
    ast.accountid = lineopp.accountid;
    ast.product2id = line.pricebookentry.product2id;
    ast.quantity += line.quantity;
    ast.price += line.total_price__c;
    ast.purchasedate = lineopp.closedate;
    ast.status = 'A6 - Closed Won';
    ast.description = line.description;
    ast.name = line.pricebookentry.product2.name;
    assets.get(lineopp.accountid).put(line.pricebookentry.product2id,ast);
  }
  asset[] updates = new asset[0];
  for(id accountid:assets.keyset())
    updates.addall(assets.get(accountid).values());
  upsert updates;
  for(opportunitylineitem line:lines)
    line.converted_to_asset__c = true;
  update lines;
}

You can just add the filter into the query (see how I did it, above).

@istest(seealldata=true)
static void test() {
  pricebook2 pb = [select id,isactive from pricebook2 where isstandard=true limit 1];
  if(!pb.isactive) {
    pb.isactive = true;
    update pb;
  }
  product2 pr = new product2(name='test',isactive=true);
  insert pr;
  pricebookentry pe = new pricebookentry(product2id=pr.id,pricebook2id=pb.id,usestandardprice=false,listprice=9.99,isactive=true);
  insert pe;
  account a = new account(name='test');
  insert a;
  opportunity o = new opportunity(name='test',closedate=date.today(),stagename='open',accountid=a.id,pricebook2id=pb.id,recordtypeid='012200000004fxZ');
  insert o;
  opportunitylineitem ol = new opportunitylineitem(opportunityid=o.id,quantity=1,pricebookentryid=pe.id,unitprice=9.99);
  insert ol;
  opportunity o2 = o.clone(false,false,false);
  insert o2;
  opportunitylineitem ol2 = new opportunitylineite(opportunityid=o2.id,quantity=2,pricebookentryid=pe.id,unitprice=9.99);
  insert ol2;
  asset aa = [select id,quantity,price,purchasedate,status,descriptoin,name from asset where accountid=:a.id];
  system.assertequals(3,aa.quantity);
  // other asserts here
}

This test method is only for positive assertion (e.g. it covers only conditions that produce code coverage). You should write similar test method for negative assertions (e.g. make sure it doesn't update assets when it's a different record type).