Enable more tests on non-Windows. (#6199)

Some tests were `#ifdef _WINDOWS` even though they compiled
and ran on other platforms. Remove the #ifdef protections
in these cases.

src/test/dl_product_relation.cpp

@@ -4,7 +4,6 @@ Copyright (c) 2015 Microsoft Corporation
 
 --*/
 
-#ifdef _WINDOWS
 #include "ast/reg_decl_plugins.h"
 #include "muz/base/dl_context.h"
 #include "muz/fp/dl_register_engine.h"
@@ -362,7 +361,3 @@ void tst_dl_product_relation() {
     test_finite_product_relation(fparams, params);
     
 }
-#else
-void tst_dl_product_relation() {
-}
-#endif