twister: coverage: set gcov working directory

gcovr runs gcov to generate temporary coverage files into the gcov
working directory. The working directory defaults to gcovr's -r
argument.

When running twister with the --coverage-per-instance argument, multiple
gcovr instances may run in parallel. Since they all share the same gcov
working directory, they may interfere with each other, resulting in
errors like this:

	AssertionError: Sanity check failed, output file
	spinlock.h##4167b923a06cc9590c8eef372f016a6d.gcov doesn't exist
	but no error from GCOV detected.

To fix this, specify a separate gcov working directory for every test
through gcovr's --gcov-object-directory option.

Signed-off-by: Marco Widmer <marco.widmer@bytesatwork.ch>