don't run test in parallel

[remicollet]

Since 8.6.0beta1

  . run-tests.php now runs in parallel by default, using up to 10 automatically
    detected workers. Pass -j1 for sequential execution. --asan, --msan, and
    Valgrind default to at most two workers.

[remicollet]

Using 3.4.0 + this patch, no failure

Using master + this patch

=====================================================================
FAILED TEST SUMMARY
---------------------------------------------------------------------
Memcached::getMulti() with bad unserialize [tests/getmulti_badunserialize.phpt]
Memcached::getDelayed() with bad unserialize [tests/getdelayed_badunserialize.phpt]
Memcached::fetch() with bad unserialize [tests/fetchall_badunserialize.phpt]
Memcached::fetch() with bad unserialize [tests/fetch_badunserialize.phpt]
=====================================================================