Use the same test_name as the DV2 result

Instead of relying on the environment variable to be the same as it was
when the result was generated, we use the same test_name as that result
when looking for the associated vcf-result
Don't look for vcf results when there aren't any

Combine results whose input results did not both make a vcf result don't
have an associated vcf result of their own.
Use the previous results test_name

It is an error if the previous_result could have a different test name
from the current result.
Ensure that the previous_result shares the test_name of this result

This is when looking up the previous_result from the current_result.
Add input_a and input_b properties