Compile warnings for some large file tests.

[edhartnett]

With large file testing turned on, I see these warings:

../../../nc_test/tst_big_var6.c:88:37: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
      printf("error on start[0..2]: %d,%d,%d  j: %d, expected %d got %d\n",
                                    ~^
                                    %ld
      start[0], start[1], start[2], j, FIRST_VAL, data[j]);
      ~~~~~~~~                        
../../../nc_test/tst_big_var6.c:88:40: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
      printf("error on start[0..2]: %d,%d,%d  j: %d, expected %d got %d\n",
                                       ~^
                                       %ld
      start[0], start[1], start[2], j, FIRST_VAL, data[j]);
                ~~~~~~~~                 
../../../nc_test/tst_big_var6.c:88:43: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
      printf("error on start[0..2]: %d,%d,%d  j: %d, expected %d got %d\n",
                                          ~^
                                          %ld
      start[0], start[1], start[2], j, FIRST_VAL, data[j]);
                          ~~~~~~~~          
../../../nc_test/tst_big_var6.c:52:15: warning: unused variable ‘k’ [-Wunused-variable]
     int i, j, k;
               ^
../../../nc_test/tst_big_var6.c:52:9: warning: unused variable ‘i’ [-Wunused-variable]
     int i, j, k;
         ^
../../../nc_test/tst_big_var2.c: In function ‘test_big_var’:
../../../nc_test/tst_big_var2.c:95:31: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
   printf("error on start[0]: %d i: %d j: %d expected %d got %d\n",
                              ~^
                              %ld
          start[0], i, j, 42, data[i][j]);
          ~~~~~~~~              
../../../nc_test/tst_big_var2.c:109:36: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t {aka long unsigned int}’ [-Wformat=]
        printf("error on start[0]: %d i: %d j: %d expected %d got %d\n",
                                   ~^
                                   %ld
        start[0], i, j, 19, data[i][j]);
        ~~~~~~~~                     
../../../nc_test/tst_big_var2.c:50:9: warning: unused variable ‘nval_in’ [-Wunused-variable]
     int nval_in;
         ^~~~~~~
../../../nc_test/tst_big_var2.c:49:9: warning: unused variable ‘nval’ [-Wunused-variable]
     int nval = 99;
         ^~~~
../../../nc_test/tst_big_var2.c:48:12: warning: unused variable ‘index’ [-Wunused-variable]
     size_t index[NUMDIMS];
            ^~~~~

All easy to fix. I will put fixes in my current open PR.