revisit the test suite

[jkowalleck]

the test suite has four targets/scenarios today:

purl-spec/PURL-SPECIFICATION.rst

Lines 473 to 485 in 65d98c4

	To test ``purl`` parsing and building, a tool can use this test suite and for
	every listed test object, run these tests:
	- parsing the test canonical ``purl`` then re-building a ``purl`` from these parsed
	components should return the test canonical ``purl``
	- parsing the test ``purl`` should return the components parsed from the test
	canonical ``purl``
	- parsing the test ``purl`` then re-building a ``purl`` from these parsed components
	should return the test canonical ``purl``
	- building a ``purl`` from the test components should return the test canonical ``purl``

discussions in multiple issues & pull requests showed, that the test cases did not properly adhere to their own cases.

• for some cases, the components are normalized/canonicalized, where the purl was not - which would just take some targets/scenarios away
• for some cases, the is_invalid was true, where actually all tests were expected to pass for all targets/scenarios
• ... to be continued ...

this shall be fixed.