Test if coverage view also shows completly uncovered css files

Make sure all files are shown even when not covered so far
.../devtools/coverage/resources/css-coverage-import.html CSS used: 0 unused: 27 total: 27
.../devtools/coverage/resources/used.css CSS used: 24 unused: 0 total: 24
Make sure files are added as they are loaded on reload
Page reloaded.
.../devtools/coverage/resources/css-coverage-import.html CSS used: 0 unused: 27 total: 27
.../devtools/coverage/resources/used.css CSS used: 24 unused: 0 total: 24