cli: Tidy up "result show" for single config and sut case

When a single config and sut is in the filtered result set, we show a
full set of summary statistics. Previously we failed to round ci95min
and ci95max columns to 2 decimal places. Let's fix that.

Additionally, when --relative is provided for the single config/sut
case, let's still show the absolute mean, then show the various other
summary stats as percentages relative to it. Previously we were showing
0% for the mean which isn't very helpful.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>