benchmark: make output RFC 4180 compliant
I had trouble importing the generated CSV into other software because it doesn't conform to RFC 4180, the de facto standard for CSV.
Relevant information from the RFC:
Spaces are considered part of a field and should not be ignored. [...] If fields are not enclosed with double quotes, then double quotes may not appear inside the fields.
Since spaces are considered part of a field, we end up with fields of the form ' "foo"'
(note the space in front of the quoted value), and now the quote isn't the first character within the field, so it violates the RFC.