tests: split tests on functions string_tolower and string_toupper

tests/unit/core/test-core-string.cpp

@@ -157,6 +157,23 @@ TEST(CoreString, Strndup)
 /*
  * Tests functions:
  *   string_tolower
+ */
+
+TEST(CoreString, ToLower)
+{
+    char *str;
+
+    str = strdup ("ABC");
+
+    string_tolower (NULL);
+    string_tolower (str);
+    STRCMP_EQUAL("abc", str);
+
+    free (str);
+}
+
+/*
+ * Tests functions:
  *   string_toupper
  */
 
@@ -164,10 +181,9 @@ TEST(CoreString, Case)
 {
     char *str;
 
-    str = strdup ("ABC");
+    str = strdup ("abc");
 
-    string_tolower (str);
-    STRCMP_EQUAL("abc", str);
+    string_toupper (NULL);
     string_toupper (str);
     STRCMP_EQUAL("ABC", str);