Fixed self-assign warning in tests

Some of the tests were creating a variable `res`, however the test
system itself relies on it's own `res` variable. This worked out by
luck, but could lead to problems if the res variables were different
types.

Changed the generated variable in the test system to the less common
name `test`, which also works out to share the same prefix as other test
functions.

tests/template.fmt

@@ -58,7 +58,7 @@ lfs_size_t size;
 lfs_size_t wsize;
 lfs_size_t rsize;
 
-uintmax_t res;
+uintmax_t test;
 
 #ifndef LFS_READ_SIZE
 #define LFS_READ_SIZE 16