EX-3409 tests: handle pcc async attach better

EX-3409 tests: handle pcc async attach better

Reset llite.*.pcc_async_threshold to the original value when the
subtest exits, so that it doesn't affect later tests.

Wait for async pcc attach to finish in test_47.

Test-Parameters: trivial testlist=sanity-pcc env=ONLY=46-47,ONLY_REPEAT=100
Test-Parameters: testlist=sanity-pcc
Test-Parameters: testlist=sanity-pcc
Test-Parameters: testlist=sanity-pcc
Test-Parameters: testlist=sanity-pcc
Test-Parameters: testlist=sanity-pcc
Test-Parameters: testlist=sanity-pcc
Test-Parameters: testlist=sanity-pcc
Test-Parameters: testlist=sanity-pcc
Test-Parameters: testlist=sanity-pcc
Test-Parameters: testlist=sanity-pcc
Fixes: 6c693d126197 ("EX-3764 pcc: avoid panic in asynchronous attach thread")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I9447fa1fc5ac20fd86340c2eda9bc08ba23ebbe5
Reviewed-on: https://review.whamcloud.com/44982
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>