Rename `math/big` test package

This is in line with the other tests, and it does not seem to affect
building the library.

tests/core/math/big/test.odin

@@ -8,7 +8,7 @@
 
 	This file exports procedures for use with the test.py test suite.
 */
-package math_big_tests
+package test_core_math_big
 
 /*
 	TODO: Write tests for `internal_*` and test reusing parameters with the public implementations.