Introduce tests/test_utils/mod.rs and use it consistently

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.

src/test_utils.rs

@@ -10,6 +10,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+/// This module contains internal utilities used for testing the library.
+/// While technically public, the library's users are not supposed to rely
+/// on this module, as it will change without notice.
+//
+// Integration tests (i.e. everything under `tests/`) import this
+// via `tests/test_utils/mod.rs`.
 use std::fmt::Debug;
 
 use super::ast::*;