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

Remove $FreeBSD$: one-line sh pattern

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

Limit gmirror failpoint tests to the test worker

This avoids injecting errors into the test system's mirrors.

gnop seems like a good solution here but it injects errors at the wrong
place vs where these tests expect and does not support a 'max global count'
like the failpoints do with 'n*' syntax.

Reviewed by: cem, vangyzen
Sponsored by: Dell EMC Isilon

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