Make get_dummy_map_from_header useful outside of core

[Cadair]

Describe the bug

Our get_dummy_map_from_header helper would be useful outside of core (I was trying to use it in sunkit-magex), but because of how it looks up the filename:

sunpy/sunpy/data/test/__init__.py

Lines 51 to 54 in 5b27fef

	if isinstance(filename, Path):
	# NOTE: get_pkg_data_filename does not accept Path objects
	filename = filename.as_posix()
	return get_pkg_data_filename(filename, package="sunpy.data.test", **kwargs)

it doesn't work unless the file is in the sunpy repo.

To Reproduce

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Installation method

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