Twister: Refactor Test(Harness).handle()

The function `handle()` from class `Test(Harness)` had most of its
code being the direct copy of the code from
`Harness.process_test(line)`. This patch replaces the copied code
with the call for `process_test(line)` and removes repetition of
this function call.

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>