This PR updates the behavior of `deno test --coverage` option. Now if
`--coverage` option is specified, `deno test` command automatically
shows summary report in the terminal, and generates the lcov report in
`$coverage_dir/lcov.info` and html report in `$coverage_dir/html/`
This change also adds `--coverage-raw-data-only` flag, which prevents
the above reports generated, instead only generates the raw json
coverage data (which is the same as current behavior)
