twister: coverage: Fix device handler coverage collection mode

Twister DeviceHandler now checks `--enable-coverage` command line argument
instead of `--coverage` when it deals with device output.
This resolves potential problem when only `--enable-coverage` argument
is given and the coverage report is not needed. In this case the test image
which is built for code coverage works slower also producing additional
console output, so the additional DeviceHandler timeout still have to be
applied and the output with coverage data correctly processed by Harness.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>