twister: coverage: Fix LCOV full path prefixes

In some cases genhtml incorrectly built reports from LCOV
coverage data using full path for some of the source files
and relative paths for other files.
This fix adds `--prefix` parameter to shorten paths explicitly
relative to the ZEPHYR_BASE directory.

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