generic/420: truncate testfile before executing the test

generic/420: truncate testfile before executing the test

If generic/437 is run before generic/420, the latter fails with:

  4c4
  < stat.size = 2048
  ---
  > stat.size = 2097152

because both use $TEST_DIR/testfile.  generic/437 leaves it at 2M,
while generic/420 assumes that it is empty (or at least smaller than
2048 bytes).

Use a private test file (testfile.$seq) and truncate it on open just in
case.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>