Move test cases from collections to atomic test files #765
Description
PURL test files are currently organized in collections for the core specification and by PURL type. Each test file contains test cases with various test_group and test_type attributes.
This issue proposes that we restructure the test files so that each test file contains a single test case.
The primary benefit is that it should be easier to assign a unique identifier to each test case as the test file name - see also #761. This change would require a deeper folder structure for the PURL type tests, but likely only one level deeper like:
/tests/types/alpm/
/tests/types/apk/
/tests/types/bazel/