[fine] Many test improvements, error improvements

- Check for more error conditions
- Move to more definitive error assertions
- Simpler error messages in some cases
- Test more conditions more thoroughly, revisit old tests

fine/tests/expression/errors/duplicates.fine

@@ -0,0 +1,16 @@
+fun nested() {
+  fun foo() {}
+  fun foo() {}
+
+  ;
+}
+
+fun nested() {}
+
+class Bar {}
+class Bar {}
+
+// @expect-errors:
+// | 3:2: duplicate definition of function 'foo'
+// | 8:0: duplicate definition of function 'nested'
+// | 11:0: duplicate definition of class 'Bar'