Introduces TestExecutionResources class to encapsulate the logic
handling binaries filesystem find and path retrieval.

atrt uses --prefix0 and --prefix1 to set MySQL installation paths from
which it finds binaries, libraries, and scripts (test execution
resources).
--prefix0 is assumed as default and paths are stored in global variables
and used across the code.
--prefix1 is assumed to be used only in upgrade tests, and switching
from --prefix0 to --prefix1 paths is done by replacing the base path and
then checking for the files.
Furthermore, when we introduced Custom Processes, the existent structure
didn't allow to accomodate such binaries and logic ended up a bit more
fragmented.

The TestExecutionResources was designed to fix all above issues.
It's pre-filled with known resources (binaries/libraries/scripts) but
allows to further register resources that could be needed during runtime
(e.g. custom processes).
The full path of all binaries (for both --prefix0 and --prefix 1) are
loaded as soon as prefixes are available and errors are signalled.
This class is then meant to be used to retrieve paths for both --prefix0
or --prefix1 either in the single-version scenario, upgrade test
scenarios, or mixed-version scenarios (to be implemented next).

Change-Id: I49fcc2d209bbf2dfb7ae3ec7e0cfdc95276753bc