clang-15 mpich_basic/adapt: unused value warning

"use" t1 in the mpich_basic adapt test to avoid clang-15 warning.

t/mpi/mpich_basic/adapt.c

@@ -1135,6 +1135,7 @@ int main(int argc, char *argv[])
     free(bwdata01);
     free(bwdata12);
     free(bwdata012);
+    (void)t1; // avoid "t1 set but not used"
     return 0;
 }