testsuite: Handle large number of kernel options

testsuite: Handle large number of kernel options

Once there are more than a certain number of kernel config options
set (this happened for us with kernel 4.17), the method of passing
those as command line arguments exceeds the maximum number of
arguments the shell supports. This causes the whole testsuite to
fail.
Instead, create a temporary file and modify its contents so that
the config option variables are exported. Then this file can be
sourced in before running the tests.

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>