Unit tests fail when using small key

[Ngoguey42]

Changing module String_size = struct let length = 20 end in

index/test/unix/common.ml

Line 11 in 4decebd

let length = 20

to a small value breaks the unit tests:

length = 3 or more doesn't fail the tests.

length = 2 sometimes fails these unit tests:

  [FAIL]        live                    3   fail add (key).
  [FAIL]        close                   1   close and reopen more.

length = 1 sometimes fails these unit tests:

  [FAIL]        close                   1   close and reopen more.
  [FAIL]        close                   2   crash and continue.
  [FAIL]        merge                   2   interleaved merge.
  [FAIL]        merge                   3   write at the end of merge.
  [FAIL]        flush_callback          2   replace.

length = 0 always fails all tests except the io_array ones