Sure, you can accomplish this by creating an Apex trigger on the Account object in Salesforce. The following is the code for the trigger:

```
trigger AccountTrigger on Account (before insert, before update) {
  // Get the random number
  Integer randomNumber = Math.floor(Math.random() * 100000);

  // Set the account number field
  if (trigger.isBeforeInsert()) {
    Account record = trigger.new;
    record.AccountNumber = 'test' + randomNumber;
  } else {
    Account record = trigger.old;
    record.AccountNumber = 'test' + randomNumber;
  }
}
```

This trigger will first get a random number between 0 and 99999. Then, it will set the account number field to "test" plus the random number. If the trigger is fired before an insert, the new account record will be updated. If the trigger is fired before an update, the old account record will be updated.

To create this trigger, you can use the following steps:

1. Go to Setup > Develop > Apex Triggers.
2. Click New.
3. In the Name field, enter a name for the trigger.
4. In the Object field, select Account.
5. In the Before field, select Insert and Update.
6. Click Save.
7. In the Trigger Editor, paste the code from above.
8. Click Save and Close.

Once the trigger is created, it will be automatically executed whenever a new or updated account record is created or updated.
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