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/

Fix geli device cleanup

Final cleanup routines shouldn't be called from testcases; it should be called
from the testcase cleanup routine.

Furthermore, `geli_test_cleanup` should take care of cleaning up geli providers
and the memory disks used for the geli providers. `geli_test_cleanup` will always
be executed whereas the equivalent logic in `geli_test_body`, may not have been
executed if the test failed prior to the logic being run.

Prior to this change, the test case was trying to clean up `$md` twice: once in
at the end of the test case body function, and the other in the cleanup function.
The cleanup function logic was failing because there wasn't anything to clean up
in the cleanup function and the errors weren't being ignored.

This fixes FreeBSD test suite runs after r345864.

PR: 237128
Reviewed by: asomers, pjd
Approved by: emaste (mentor)
MFC with: r345864
Differential Revision: https://reviews.freebsd.org/D19854

Implement tests for online expansion:
- init, init -R
- onetime, onetime -R
- 512 and 4k sectors
- encryption only
- encryption and authentication
- configure -r/-R for detached providers
- configure -r/-R for attached providers
- all keys allocated (10, 20 and 30MB provider sizes)
- keys allocated on demand (10, 20 and 30PB provider sizes)
- reading and writing to provider after expansion (10-30MB only)
- checking if metadata in old location is cleared.

Obtained from: Fudo Security