ktest: Add eval '=~' command to modify variables in config file

ktest: Add eval '=~' command to modify variables in config file

With the added variable ${KERNEL_VERSION}, it is useful to be
able to use parts of it for other variables.

For example, if you want to create a warnings file for each major
kernel version to test sub versions against you can create
your warnings file with like this:

  WARNINGS_FILE = warnings-file-${KERNEL_VERSION}

But this may add 3.8.12 or something, and we want all 3.8.* to
use the same file, and 3.10.* to use another file, and so on.
With the eval command we can, by adding:

  WARNINGS_FILE =~ s/(-file-\d+\.\d+).*/$1/

Which will chop off the extra characters after the 3.8.

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