HPCC-11629 Regression suite should support --target all

Add the requested parameter to set and run subcommands.

Update built-in help and README.rst.

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>
Merge pull request #6115 from AttilaVamos/HPCC-11629-improvement

HPCC-11629 Regression suite should support --target all

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>