ADBDEV-8304: Cleanup never terminates helper processes

[Stolb27]

Cleanup never terminates helper processes

The problem is caused by additional grep command that was introduced in
4bfd4c0. Expressions starting with -- should be escaped properly to
returns anything. The command pipeline is failed with:

grep: unrecognized option '--pipe-file

on stderr and empty stdout. As a result, process cleanup do nothing.

This patch add expression separators for grep command.

Ticket: ADBDEV-8304

[Stolb27]

To ensure that the test from commit 4bfd4c0 fails without your patch, you need to add an error check to the CheckHelperPids function, as is done in the similar GetHelperFileCount function.

In my opinion, it's a separate problem to implement exit-code handling.