Add default formatter to rspec if none given

What does this MR do and why?

Quarantine E2E and non-E2E tests using Quaranti... (!218566 - merged) added a formatter to our default RSpec configuration. RSpec only adds the default formatter (progress) if no formatters were added to the config. As a result, our RSpec is now very quiet (though still running the tests!):

$ bin/rspec spec/lib/banzai/color_parser_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

The specs actually ran (you'll get a non-zero exit on failure), but we don't get any output. Explicitly specifying a formatter has the effect of adding it manually:

$ bin/rspec spec/lib/banzai/color_parser_spec.rb -fp
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}
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Finished in 0.02176 seconds (files took 1.61 seconds to load)
97 examples, 0 failures

This MR explicitly adds the default formatter if none was specified on the command line, so the later addition of GitlabQuality::TestTooling::TestQuarantine::QuarantineFormatter doesn't affect the default output.

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