MyTestClass.cls

@isTest
public class MyTestClass {
    static testMethod void testMyMethod() {
        List<Account> accounts = new List<Account>{
            new Account(Name='Test Account 1'),
            new Account(Name='Test Account 2')
        };
        insert accounts;
        Test.setFixedSearchResults(accounts);
        List<Account> results = [FIND 'Test Account*' IN ALL FIELDS RETURNING Account(Name)];
        System.assertEquals(2, results.size());
    }
}