Groups make it easier to get an overview of the coverage without having
to scan through a single very long list of files. They also display a
possibly helpful per-group coverage.

The project name is normally derived from the `root` directory which
happens to be `Library` in our case, thus make it explicitly `Homebrew`.

Closes Homebrew/homebrew#49303.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

While running a command from the integration tests, invoke `.result` to
trigger a save of the result set, but don't call `.format!` to avoid
(re)generating the HTML output with every run. The final output will
still be written once the main unit test run completes.

This significantly speeds up the integration tests, that take about 1/3
less time to complete when not generating the intermediate output.

Closes Homebrew/homebrew#48280.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Commands executed during integration testing are executed in a separate
process and thus generate a new result set for each command. To avoid
that these results override each other, they need to have a unique
`command_name`. Derive this name from the test class/name and the index
of the command inside that test, resulting in identifiers like
`IntegrationCommandTests#test_prefix.1 brew --prefix`.

Also replaces `TEST_TMPDIR` in the arguments with `"@TMPDIR@"` to get a
cleaner command identifier that is independent of the temporary
directory that changes with every run.

The filter for `vendor/bundle/` is useless because this directory is
located in `Homebrew/test/` and that one is already filtered. Moreover,
SimpleCov already loads the `bundler_filter` profile (that installs
basically the same filter) in its default configuration.

Closes Homebrew/homebrew#47185.

Signed-off-by: Xu Cheng <xucheng@me.com>