combine VENDOR_TEST_BRANCHES and VENDOR_TEST_SHAS into one param

Currently, for running vendor repo tests, users can provide VENDOR_TEST_BRANCHES or VENDOR_TEST_SHAS. To reduce the parameters that we have, we combine VENDOR_TEST_BRANCHES and VENDOR_TEST_SHAS into one parameter. That is, keep VENDOR_TEST_BRANCHES and allow users to set branches, tags, or SHA into this parameter.

Start by locating all references to VENDOR_TEST_BRANCHES and VENDOR_TEST_SHAS in the vendor repository test configuration and documentation. Determine how branches, tags, and SHAs are parsed today, then update the configuration so VENDOR_TEST_BRANCHES accepts all three forms and remove the separate parameter. Done means vendor tests work with each supported reference and existing tests pass.

Written by the indexing model from the issue text.