oeqa/runtime: Add case, context and loader classes for runtime testing

oeqa/runtime: Add case, context and loader classes for runtime testing

This adds OERuntimeTestCase, OERuntimeTestContext, and OERuntimeTestLoader
to be used for runtime testing.

As expected there are some changes in runtime context:

- Adds the target to be used for runtime testing, the default
is a SSH connection to the device under test running a OE image.
- Runtime context requires image manifest because several
tests are skipped if a package is missing or installed.
- Several tests require the output of the ps command and it changes
its output and arguments if busybox o procps is installed, so the
case must use the correct ps command.

[YOCTO #10234]

(From OE-Core rev: f995f178de79d6d11422cd879d06371811f50651)

Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>