WL#5050: Increase coverage of Field::val_xxx methods in main test suite

Coverage analysis has shown that a few val_xxx methods in Field classes are not
covered by the main test suite.  This worklog will extend test suite to cover
those methods.

The following val_xxx methods is not covered by the main test suite.  Tests for
these methods should, if possible, be added:

Field_double::val_int()
Field_float::val_int()
Field_blob::val_int()
Field_varstring::val_int()

In addition, the following classes are not covered at all:
Field_null
Field_date
Field_decimal

An attempt has been made to find SQL that invoke the methods in question.
Result:

Field_double::val_int()
Field_float::val_int()
  To test these methods, test cases that cast of double/float columns to
  signed/unsigned integers will be added to main.cast test.  

Field_blob::val_int()
Field_varstring::val_int()
  The following statements will trigger these methods:

  select 1 from t1 where ;
  select left('1234567890', ) from t1;    

  Similar test cases will be added to main.type_blob test.  
  (Casting relevant columns to integers as above do not apply since
  conversion is then done by Item_func_signed::val_int_from_str())


Field_null
  So far, no way has been found to instantiate Field_null with SQL.

Field_date
Field_decimal
  These classes represent obsolete SQL types and will not be tested.
  A worklog will be created for the removal of these classes.