Support absolute value thresholds in the reference tuple

[vkarak]

Currently the thresholds are essentially percentages relative to the reference value. As a result, reference values cannot be 0, as also the bounds will be set to zero and if done so, reframe will ignore them. See #2833. We need a way to support absolute thresholds as well as relative.

[jack-morrison]

I have an interest in 0 reference values for some tests, so I'll take a look at this one.

[vkarak]

I think we should simply treat zero references specially and in this case interpret the bounds as absolute.