twister: try to clean ninja zombie

sometimes ninja fails to launch qemu, the sub-thread
can't read anything from qemu pipeline, then the
corresponding testcase will timeout. Then the
sub-thread will get blocked if it call join()

set terminate as the Handler's method, then
Handler's children class can call it

Signed-off-by: Jingru Wang <jingru@synopsys.com>
1 file changed