twister: support parallel coverage with lcov >=2.0

lcov 2.0 added support for processing coverage data in parallel, which
provides a large speedup when processing many files, at the cost of some
additional overhead. When running the Chrome EC tests with coverage,
parallel reporting on a 36C72T machine reduces the time spent generating
coverage reports by 40 minutes (from approximately 1 hour to 20 minutes
total runtime), at the cost of about 3x greater CPU time overall
(assumed to be overhead for parallel processing, likely from spawning
much larger numbers of subprocesses).

The level of lcov parallelism is taken from the --jobs option passed to
twister, allowing lcov to choose if unspecified.

Signed-off-by: Peter Marheine <pmarheine@chromium.org>