If you are asking for an Apex trigger code that creates a new account number for every account record with the prefix test to be test concatenated with a random number. This is an example:

trigger GenerateAccountNumber on Account (before insert) {
    // Iterate over each account record in the trigger context
    for (Account acc : Trigger.new) {
        // Check if the account number is null or blank and the account name contains the word "test"
        if (String.isBlank(acc.AccountNumber) && acc.Name.toLowerCase().contains('test')) {
            // Generate a random number between 100 and 999
            Integer randomNum = (Integer)Math.floor(Math.random() * (999 - 100 + 1) + 100);
            // Set the new account number with the prefix "test" and the random number
            acc.AccountNumber = 'test' + randomNum;
        }
    }
}
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