CTest follow-up: add flag to control old diags handling

[EZoni]

As noticed by some developers, our build directories start to bloat to 100s of GB, because when we run a test multiple times (without removing the build directory) by default any exisisting diagnostics output is renamed as "old" and kept in place (rather than deleted). This is likely performed by calling the function amrex::UtilCreateCleanDirectory somewhere in AMReX or WarpX.

Action items that came up while discussing this offline:

• Add a flag in AMReX or WarpX to control this (new ParmParse option)
• When we run tests, delete old diags files instead of renaming them