internal: Add test module (#2432)

This has been sitting in #2342, which likely won't get merged, so adding
it here.

Author: max-sixty

prql-compiler/src/tests/mod.rs

@@ -1,4 +1,5 @@
 mod test;
 mod test_bad_error_messages;
+mod test_error_messages;
 
 pub use test::compile;