pf tests: Improve assertion messaging of mbuf tests

[igoro]

A minuscule improvement to get the following upon atf_check_equal failure:

Error Message:
1 != $(jexec alcatraz sysctl -n net.dummymbuf.hits) (1 != 2)

instead of this:

Error Message:
1 != 2 (1 != 2)

It's still non-ideal for these specific test cases, but reveals a best practice if any copy-pasting is needed in the future.

Wanted to do it like a year ago. I'm thinking to improve the atf manual page to make it clear (if possible), right now it presents it with double quotes which might end up being used such way. But if we check the code behind then it's obvious that shell expansion was intended to be left for later.