Annotate qa/457 into the flakey group, its time windows are racey
pmatop.py: fix help keys

From pmatop(1):

  Miscellaneous interactive commands:

    ?  Request for help information (also the key ’h’ can be pressed).
pmcd_wait now responds to ENOENT.

ENOENT indicates that the pmcd's unix domain socket file has
not yet been created. pmcd_wait will try again, up to the configured
maximum number of times.

This commit also reverts commit
d62b31c5282c6133c87d067db28b3cd440f650f3 which is no longer
necessary.