docs: Add CLAUDE.md testing guide for prqlc tests (#5612)

Co-authored-by: Claude <noreply@anthropic.com>

Author: max-sixty

prqlc/prqlc/tests/CLAUDE.md

@@ -0,0 +1,31 @@
+# Tests
+
+## Structure
+
+- **`integration/sql.rs`** — Unit tests for PRQL→SQL compilation. Fast, focused,
+  preferred for most changes.
+- **`integration/queries/*.prql`** — Integration tests generating 6 snapshots
+  each (different SQL dialects). Use sparingly.
+- **`integration/error_messages.rs`** — Tests for compiler error diagnostics.
+- **`integration/dbs/`** — Database runners for executing queries against real
+  databases.
+
+## Guidelines
+
+**80% unit tests, 20% integration tests.** Integration tests in
+`integration/queries/` are verbose — each `.prql` file generates six snapshot
+files. Prefer unit tests in [`integration/sql.rs`](./integration/sql.rs):
+
+```rust
+#[test]
+fn test_my_feature() {
+    assert_snapshot!(compile(r#"
+    from foo
+    select bar
+    "#).unwrap(), @"...");
+}
+```
+
+New integration test files are appropriate when validating against real
+databases or testing behavior across compilation stages. Extend existing tests
+when possible.