1.0.39.17: Reduce stress level of sb-concurrency tests.

1.0.39.17: Reduce stress level of sb-concurrency tests.

Reduce the stress level of the MAILBOX tests by a factor of 10.

Reasons:

  a) As contrib tests are run as part of a normal SBCL build, the
     mailbox tests are executed each time SBCL is built. And they
     really result in a few minutes of 100% cpu usage and high load
     averages because of many simultaneously running threads.

  b) On BSD platforms, the tests were reported several times in past
     to fail. From what I'm told (and if I remember correctly), the
     issue is, however, not necessarily concurrency bugs, but a
     coarser SLEEP granularity than on Linux platform.