twister: Fix reported testcase execution time

Testcase execution time doesn't match between twister.xml and
twister.log. Testcase execution time is the sum of the previous
testcases' execution time plus its own execution time in
twister.xml.

This patch fixes the issue above.

Signed-off-by: Ioannis Damigos <ioannis.damigos.uj@renesas.com>