rc/tests: Fix process flags checks using ps(1)

These checks use ps(1) with both the '-p' and '-a' flags, which worked
so far as ps(1)'s '-a' option was buggy. Since the fix in commit
"ps(1): Make '-a' and '-A' always show all processes"
(93a94ce731a89b56), such a command-line always selects all processes,
making the specified '-p' useless and the test wrong. Remove the
useless '-a'.

While here, remove the redundant '-x', as '-p' implies '-x' by default.

MFC after: 1 day
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50201