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_common.rs

@@ -19,14 +19,13 @@
 //! dialect-specific parsing rules).
 
 #[macro_use]
-#[path = "macros/mod.rs"]
-mod macros;
+mod test_utils;
+use test_utils::{all_dialects, expr_from_projection, join, number, only, table};
 
 use matches::assert_matches;
 use sqlparser::ast::*;
 use sqlparser::dialect::keywords::ALL_KEYWORDS;
 use sqlparser::parser::ParserError;
-use sqlparser::test_utils::{all_dialects, expr_from_projection, join, number, only, table};
 
 #[test]
 fn parse_insert_values() {