refactor: Remove outdated (and unused) default repo_top argument

[rswarbrick]

This code was based on dvsim living inside the OpenTitan repository. Fortunately, it is never used: we always pass a third argument to _parse_testplan.

Also, fail more understandably if there is an hjson parse error or an invalid filename passed to _parse_hjson.

Finally, simplify the Testplan constructor so that it always expects a name for its testplan (rather than exiting the program silently if there isn't one).

We also change that argument to explicitly have type str (as opposed to Path) and explain why: the string can have suffix values that filter the set of tests.

[AlexJones0]

This was a problem in the original code, but we should probably fix it while we're here.

Reference counting probably means that this issue will not be a problem, but we should be explicitly closing the file handle after opening it rather than leaving it open.

A better approach would be:

Suggested change

	"""Parse an hjson file at the given path and return it as a dict."""
	return hjson.load(Path(filename).open())
	"""Parse an hjson file at the given path and return it as a dict."""
	with Path(filename).open("r", encoding="utf-8") as f:
	return hjson.load(f)

Or maybe even better as:

Suggested change

	"""Parse an hjson file at the given path and return it as a dict."""
	return hjson.load(Path(filename).open())
	"""Parse an hjson file at the given path and return it as a dict."""
	return hjson.loads(Path(filename).read_text(encoding="utf-8"))

(to let Pathlib handle the read file mode for us).

[AlexJones0]

Suggested change

	filename: Describes the HJson file that captures the testplan. This
	filename: Describes the Hjson file that captures the testplan. This

(a couple more times in these changed docs - I appreciate it was in the original but it'd be nice to be consistent).

[AlexJones0]

The explanation is great, but given that we later do

        filename = Path(split[0])
        tags = set(split[1:])

I'd suggest the name isn't great anyway and we should rename it. Perhaps something like:

• tagged_filename
• filename_with_tags
• file_spec / testplan_file_spec

etc. etc. - take your pick.