To share helper macros between various tests/* we added a new module
(tests/macros/mod.rs). This made the prologue to be used in tests quite
long and a little weird:
```
#[macro_use]
#[path = "macros/mod.rs"]
mod macros;
use sqlparser::test_utils::*;
```
This simplifies it to:
```
#[macro_use]
mod test_utils;
use test_utils::*;
```
- and switches all existing tests to the new prologue simultaneously...
...while fixing a few other inconsistencies and adding a few comments
about the way `test_utils` work.
