tests: twister: robot: Fix setting testcases status

Currently, when a Robot test is run, the test case status value is
reported as `None`, which causes the XML to contain information about
the test being skipped due to misconfiguration.

This commit fixes the value reported in Twister XML test result by
assigning the test result to the `status` variable of a testcase
instance after the test has finished running.

Signed-off-by: Franciszek Pindel <fpindel@internships.antmicro.com>
Signed-off-by: Piotr Zierhoffer <pzierhoffer@antmicro.com>