Streamlined the handling of 'exectimeout' in sub_test.

sub_test used 'exectimeout' - which indicates test execution timeout -
where it meant compilation timeout.

I fixed this by introducing 'comptimeout' and using it instead
of 'exectimeout' where appropriate.
This also allowed me to remove an unnecessary initialization
of 'exectimeout' at the top of the 'for testname' loop.

At the moment, the 'comptimeout' variable is unnecessary - the code
that reads its value is invoked only when there is no timeout,
so it is always False there.

So this change can be made even smaller by eliminating 'comptimeout'
altogether and replacing its uses with False. Any preferences? How to test this?