test: improve coverage for data_sources

[mastersans]

i'll update the issue with further details:

• rsd_source test not present, i haven't investigated this related PR for now feat: Add Red Hat data source (Fixes #2331, #2367) #2368
• nvd api 2 test having problem in previous attempts i2334
• testing download_epss_data() in epss_source if for case where epss_scores-current.csv doesn't exist.
• some others in osv source(will get more details)
• for redhat_source cases other then incremental update(line 101) and exception blocks not being covered.
• some minor cases in gad_source see codecov