REGTESTS: list all skipped tests including 'feature cmd' ones

REGTESTS: list all skipped tests including 'feature cmd' ones

The script for running regression tests is modified to improve the
visibility of skipped tests.

Previously, the reasons for skipping tests were only visible during the
test discovery phase when grepping the vtc (REQUIRE, EXCLUDE, etc).
But reg-tests skipped by vtest with the 'feature cmd' keywords were not
listed.

This change introduces the following:
  - vtest does not remove the logs itself anymore, because it is not
    able to let the log available when a test is skipped. So the -L
    parameter is now always passed to vtest
  - All skipped tests during the discovery phase are now logged to a
    'skipped.log' file within the test directory
  - The script now parses vtest logs to find tests that were skipped
    due to missing features (via the 'feature cmd' in .vtc files)
    and adds them to the skipped list.