Unbreak geli/gmirror testcases if their geom classes cannot be loaded

The problem with the logic prior to this commit was twofold:

1. The wrong set of idioms (TAP-compatible) were being applied to the ATF testcases when run, resulting in confusing ATF failure results on setup.
2. The cleanup subroutines were broken when the geom classes could not be loaded as they exited with 0 unexpectedly.

This commit changes the test code to source the class-specific configuration
(conf.sh) once globally, instead of sourcing it per testcase and per cleanup
subroutine, and to call the ATF-specific setup subroutine(s) inline in
the testcases.

The refactoring done is effectively a no-op for the TAP testcases, modulo
any refactoring done to create common code between the ATF and TAP
testcases.

This unbreaks the geli testcases converted to ATF in r327662 and r327683,
and the gmirror testcases added in r327780, respectively, when the geom
class could not be loaded.

tests/sys/geom/class/mirror/...

While here, ignore errors when turning debug failpoint sysctl off, which
could occur if the gmirror class was not loaded.

Submitted by: ngie
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd/pull/241