gh-106368: Argument clinic: improve coverage for self.valid_line() calls

[AlexWaygood]

The early return in self.state_modulename_name() is unreachable, as state_modulename_name() is only ever called from state_dsl_start, and the first thing state_dsl_start does is check whether the line is valid:

cpython/Tools/clinic/clinic.py

Lines 4655 to 4657 in 321f0f7

	if not self.valid_line(line):
	return

There are two other if not self.valid_line(line) branches in clinic.py which are reachable, but are uncovered:

cpython/Tools/clinic/clinic.py

Lines 5323 to 5330 in 321f0f7

	def state_function_docstring(self, line: str) -> None:
	assert self.function is not None
	if self.group:
	fail(f"Function {self.function.name!r} has a ']' without a matching '['.")
	if not self.valid_line(line):
	return

cpython/Tools/clinic/clinic.py

Lines 5304 to 5306 in 321f0f7

	def state_parameter_docstring(self, line: str) -> None:
	if not self.valid_line(line):
	return

Add a test that covers both of them.

• Issue: Increase Argument Clinic test coverage #106368

[erlend-aasland]

Nice, thanks!