Added tests in particular for non-selects to
improve test coverage on index_merge.
          
The test-cases include the following:
            
     1) Statements with combinations of comparison operators
        such as <, >, =, <>, is null, is not null, <=>
       
     2) Statement with ORDER BY
            
     3) Statements to verify force/use/ignore index
            
     4) multi column indexes
            
     5) sub queries
      
     6) statement with IN predicates
            
The above test-cases are present for all the non-selects