scripts: twister: clarify output directory renaming message

Change "Renaming output directory to" message to "Renaming previous
output directory to" to make it clearer that twister is renaming the
existing directory, not the current output directory.

This helps avoid confusion for new twister users who might think the
current output is going to the renamed directory.

Signed-off-by: Afonso Oliveira <afonsoo@synopsys.com>