Testing SOSL

Use       Test.setFixedSearchResults

Class

public class MySearchClass{
    public Static void doSearch(String search){
        List<List<Sobject>> searchResult;
        searchResult=[FIND :search IN ALL FIELDS RETURNING Account];
        if(searchResult!=null){
            System.debug('in if'+searchResult);
            searchResult=[FIND :search IN ALL FIELDS RETURNING Account];
        }
        else{
            System.debug('in else'+searchResult);
            searchResult=[FIND :search IN ALL FIELDS RETURNING Account];
        }
    }
}

Test Class

@isTest
public class TestMySearchClass{
    private static testMethod void testMeth(){
        Account acc = new Account(Name='testBaljeet');
        insert acc;
        Test.setFixedSearchResults(new List<Id>{acc.id});
        MySearchClass.doSearch('test');
    }
}