tcctest: One more signed zero test

This also checks that -(-0.0) is +0.0.

tests/tcctest.c

@@ -1721,6 +1721,12 @@ void prefix ## signed_zeros(void) \
             1.0 / x != 1.0 / p);\
   else\
     printf ("x != +y; this is wrong!\n");\
+  p = -y;\
+  if (x == p)\
+    printf ("Test 1.0 / x != 1.0 / -y returns %d (should be 0).\n",\
+            1.0 / x != 1.0 / p);\
+  else\
+    printf ("x != -y; this is wrong!\n");\
 }\
 void prefix ## test(void)\
 {\