ktest.pl: Add shell commands to variables

Allow variables to execute shell commands. Note, these are processed when
they are first seen while parsing the config file. This is useful if you
have the same config file used for multiple hosts (as they may be in a git
repository).

 HOSTNAME := ${shell hostname}
 DEFAULTS IF "${HOSTNAME}" == "frodo"

Link: https://lkml.kernel.org/r/20221207212944.277ee850@gandalf.local.home



Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>