How to write a test class for the SOSL Query in Salesforce

             If you do not want the query to return an empty list of results, you can use the Test.setFixedSearchResults system method to define a list of record IDs that are returned by the search.

Apex Class:

1. public class SOSLController
2. {
3.     public static void findMatchingData(String str)
4.     {
5.         List<List<Sobject>> searchList = [FIND :str RETURNING Account(Id, Name), Lead];       
6.         List<Account> accounts = ((List<Account>)searchList[0]);
7.         if(accounts.size() > 0)
8.         {
9.             for(Account currAcc : accounts)
10.             {
11.                 // do your logic.
12.             }
13.         }
14.     }
15. }

Test Class:

1. @isTest
2. private class TestSOSLController
3. {
4.     static testMethod void insertAccount()
5.     {
6.         Account acc = new Account();
7.         acc.Name = 'Test Account';
8.         insert acc;
9.        
10.         Id [] fixedSearchResults= new Id[1];
11.         fixedSearchResults[0] = acc.Id;
12.         Test.setFixedSearchResults(fixedSearchResults);
13.         SOSLController.findMatchingData('Test');
14.     }
15. }

Step 1: Create a record that you want to return in a search result.

Step 2: Create a list and assign the inserted id. 

Step 3: Use standard method Test.setFixedSearchResults and pass the above ID list.

Step 4: Call your apex method to test your functionality. 

Reference: Adding SOSL Queries to Unit Tests