EX-5176 tests: re-enable sanity test_400a

EX-5176 tests: re-enable sanity test_400a

Re-enable test_400a after it was disabled due to json-c dependency
during build test.  This library is now installed on client nodes.

Test-Parameters: trivial testlist=sanity env=ONLY=400 clientdistro=sles15sp3
Test-Parameters: testlist=sanity env=ONLY=400 clientdistro=ubuntu2004
Test-Parameters: testlist=sanity env=ONLY=400 clientdistro=el8.5
Fixes: fbfd2d0755 ("EX-5176 pcc: use JSON string for trusted.pin xattr")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I3daca219cedb447127f9402a79e6124afb3ebbe5
Reviewed-on: https://review.whamcloud.com/47363
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng, Lei <flei@whamcloud.com>