coverage tool: also save coverage summary for lcov

[fuzzah]

For golang we save coverage summary file, for lcov we don't.
Summary file is easier to store and easier to parse by external tools.

[fuzzah]

Having fuzzed lots of different apps I've come to conclusion we'd need more flexibility regarding these options. Some programs are that huge, so it takes a lot of time to create a coverage report be it HTML or TXT. Forcefully creating both reports would waste a lot of time.
User should be in charge of what reports to create, e.g. by specifying some option with available choices: html, txt, both