[LIT] Print discovered tests and percentages (#66057)

This patch adds "nice-to-have" feature in lit.
it prints the total number of discovered tests at the beginning. It is
covenient to see the total number of tests and avoid scrolling up to the
beginning of log.

Further, this patch also prints %ge of tests.

Reviewed By: RoboTux, jdenny-ornl

Co-authored-by: Madhur A <madhura@nvidia.com>