[red-knot] Unify setup_db() functions, add TestDb builder (#14777)

## Summary

- Instead of seven (more or less similar) `setup_db` functions, use just
one in a single central place.
- For every test that needs customization beyond that, offer a
`TestDbBuilder` that can control the Python target version, custom
typeshed, and pre-existing files.

The main motivation for this is that we're soon going to need
customization of the Python version, and I didn't feel like adding this
to each of the existing `setup_db` functions.

crates/red_knot_python_semantic/src/symbol.rs

@@ -90,7 +90,7 @@ impl<'db> Symbol<'db> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::types::tests::setup_db;
+    use crate::db::tests::setup_db;
 
     #[test]
     fn test_symbol_or_fall_back_to() {