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

[romainmenke]

We can't check for test coverage for this bit of code because sugarss always exists when running the tests here.

[RyanZim]

Yeah. Not sure if it's best to ignore it vs. just having the coverage report indicate that we're not actually testing this.

[romainmenke]

also unsure, coverage in itself isn't a clear signal.
100% doesn't mean that everything is tested.
It's just another tool to help write better tests.