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

[olce]

Summary

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 '-p'.

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