In-memory tables don't support `DECIMAL(...) UNIQUE KEY`

[jycor]

UNIQUE KEYs over DECIMAL types don't throw unique key error.
This is specific to in-memory tables.

enginetest:

{
	Name: "decimal unique keys",
	SetUpScript: []string{
		"create table t (i int primary key, d decimal(10, 2) unique);",
		"insert into t values (1, 1);",
	},
	Assertions: []queries.ScriptTestAssertion{
		{
			Query:       "insert into t values (2, 1);",
			ExpectedErr: sql.ErrUniqueKeyViolation,
		},
	},
},