tests: don't run atf_* in a subshell

Shell limitation is that a classic function call via $() is a subshell
and atf-sh(3) commands won't work as epxected there. Subsequently,
atf_skip inside a function won't skip a test. The test will fail later.

A working approach is to pass desired variable name as argument to
a function and don't run subshell.

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D42646
Fixes: ea82362219ee715cfbb195b2114e73fdc8599fa5

tests: Skip all tests that require mdconfig when /dev/mdctl missing

When run in a jail, /dev/mdctl is missing. So skip any tests that use
mdconfig or mdmfs with md in this case: they can't possibly work. This
is in line with other tests that test for presence of required features
and skip if they aren't present. I did this instead of checking for
jails so they can still run in jails that allow creation of md devices.

Sponsored by: Netflix

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Make indentation consistent with other tests, and use syncwait.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon

Make test scripts under tests/... non-executable

Executable bits should be set at install time instead of in the repo.
Setting executable bits on files triggers false positives with Phabricator.

MFC after: 2 months

Add regression tests for r317712 and r306743.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon