Add `sql/expression/literal_test.go`

[juanjux]

Just noticed that the module is missing the test module.

[erizocosmico]

Do you mean that file is missing?

[juanjux]

Yes, should have an specific test module for the stuff there, I guess.

[erizocosmico]

Not really, it just returns the literal kept in the struct. I don't think it's worth testing.

[juanjux]

Fair enough, I'll add a test later if I see how I can evaluate the VarChar length and return an error in NewLiteral.