Add inline-comparison as acceptance test

[wagoodman]

• Updated the test/inline-compare python script to function with the latest JSON output
• Added inline-compare step to the acceptance test pipeline, which runs multiple images (currently "debian:10.5", "centos:8.2.2004", "alpine:3.12.0", a vulnerable java image, vulnerable python image, and vulnerable alpine image ) against both Anchore engine inline-scan against syft and ensure a lower threshold in discovered vulnerabilities is met.
• Updates the RPM matcher to search for vulnerabilities against the sourcerpm (if there is one)

Note: there are two discrepancies found that lower the test result values and I've added comments that explain these discrepancies. I've also added a upper-threshold for these exceptions such that when future enhancements are put in place the test will fail as an indication that the lower threshold should be bumped.

Example run: https://github.com/anchore/grype/runs/963498059?check_suite_focus=true

Note: go.mod needs to be updated once anchore/syft#130 is merged.