Fix WIN32 build failure in defragmenter_test

Windows doesn't have setenv(), so use putenv() instead.

Change-Id: I1ec849b0b7a7112b37620a4de5c37f0499a0640d
Reviewed-on: http://review.couchbase.org/44508
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>