Rework SassSpecTest to read hrx and identify specs in the same way than the official runner

[stof]

Currently, we rely on the sass-spec convention of separating tests with a delimiter comment to identify the different specs. This causes issues when the convention is not properly respected (see #254), which happens as there is no linter enforcing the convention in sass-spec for now. We should instead read the hrx file properly to identify files and folders in it, and identify specs based on folders containing a input.scss or input.sass file (and then skipping the ones with input.sass as we don't support the sass syntax).

This can either be done adhoc in SassSpecTest or by implementing a HRX reader and then using it in SassSpecTest