add c8 test coverage

[romainmenke]

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	99.48	99.26	100	99.48
postcss-import	100	100	100	100
index.js	100	100	100	100
postcss-import/lib	98.87	98.49	100	98.87
assign-layer-names.js	100	100	100	100
data-url.js	100	100	100	100
join-layer.js	100	100	100	100
join-media.js	100	100	100	100
load-content.js	100	100	100	100
parse-statements.js	97.67	96.66	100	97.67	151-152,165-166
process-content.js	100	100	100	100
resolve-id.js	100	100	100	100