test/rgw/posix: free the quota handler in TestDriver

test/rgw/posix: free the quota handler in TestDriver

TestDriver::init() allocates quota_handler via
RGWQuotaHandler::generate_handler() but nothing frees it. The real
POSIXDriver frees it in finalize(), which the unit tests never call, so
every fixture that runs init() leaks the handler and the stat caches
hanging off it: 274 allocations, ~40KB, all rooted at generate_handler()
under ASan:

  ==6102==ERROR: LeakSanitizer: detected memory leaks
  Direct leak of 3200 byte(s) in 5 object(s) allocated from:
    #1 RGWQuotaHandler::generate_handler(...) src/rgw/rgw_quota.cc:989
    #2 TestDriver::init(...) src/test/rgw/test_rgw_posix_driver.cc:1100
    #3 POSIXDriverTest::SetUp() src/test/rgw/test_rgw_posix_driver.cc:1191
    ...
  SUMMARY: AddressSanitizer: 40099 byte(s) leaked in 274 allocation(s).

So free it in ~TestDriver(), the counterpart to the init() allocation.
~POSIXDriver() is empty and nothing else touches quota_handler, so there
is no double free, and free_handler(nullptr) is a no-op when init()
bailed out early.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>