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.

tests/sqlparser_mysql.rs

@@ -11,13 +11,15 @@
 // limitations under the License.
 
 #![warn(clippy::all)]
-
 //! Test SQL syntax specific to MySQL. The parser based on the generic dialect
 //! is also tested (on the inputs it can handle).
 
+#[macro_use]
+mod test_utils;
+use test_utils::*;
+
 use sqlparser::ast::*;
 use sqlparser::dialect::{GenericDialect, MySqlDialect};
-use sqlparser::test_utils::*;
 use sqlparser::tokenizer::Token;
 
 #[test]