The failing reason was inconsistent truncation rules: the value of virtual
column could have been evaluated to '2000' sometimes instead of '0000' for
value 'a'.

The reason why `c YEAR AS ('aaaa')` was not evaluated same is that len=4 is
a special case insidew Field_year::store.

The correct fix is: always evaluate a bad value to 0000 instead 2000.
The truncated values should be evaluated as usual.

$support_virtual_index is finally changed to 1 in gcol.gcol_ins_upd_innodb,
which is also enough for testing.

The test from original bug report is also added.