Fix test reference to index.css

I was trying to make a PR a got an issue about that index.css reference. This fix works for me & seems logical.

Before this fix I got this
```
~/Sync/Development/forks/components-flex-embed master*  
❯❯ npm run build-test

> suitcss-components-flex-embed@1.7.1 build-test /Users/MoOx/Sync/Development/forks/components-flex-embed
> npm run setup && npm run preprocess-test


> suitcss-components-flex-embed@1.7.1 setup /Users/MoOx/Sync/Development/forks/components-flex-embed
> npm install && mkdir -p build


> suitcss-components-flex-embed@1.7.1 preprocess-test /Users/MoOx/Sync/Development/forks/components-flex-embed
> suitcss test/test.css build/test.css


       error · Error: Cannot find module './index.css' from '/Users/MoOx/Sync/Development/forks/components-flex-embed/test'
             ·   at Function.module.exports [as sync] (/Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework-suit/node_modules/rework-npm/node_modules/resolve/lib/sync.js:32:11)
             ·   at resolveImport (/Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework-suit/node_modules/rework-npm/index.js:63:28)
             ·   at getImport (/Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework-suit/node_modules/rework-npm/index.js:34:20)
             ·   at /Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework-suit/node_modules/rework-npm/index.js:25:44
             ·   at module.exports (/Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework-suit/node_modules/rework-npm/node_modules/concat-map/index.js:4:17)
             ·   at inline (/Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework-suit/node_modules/rework-npm/index.js:23:23)
             ·   at /Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework-suit/node_modules/rework-npm/index.js:97:16
             ·   at Rework.use (/Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework/index.js:49:3)
             ·   at /Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework-suit/index.js:36:8
             ·   at Rework.use (/Users/MoOx/Sync/Development/forks/components-flex-embed/node_modules/suitcss-preprocessor/node_modules/rework/index.js:49:3)


npm ERR! suitcss-components-flex-embed@1.7.1 preprocess-test: `suitcss test/test.css build/test.css`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the suitcss-components-flex-embed@1.7.1 preprocess-test script.
npm ERR! This is most likely a problem with the suitcss-components-flex-embed package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     suitcss test/test.css build/test.css
npm ERR! You can get their info via:
npm ERR!     npm owner ls suitcss-components-flex-embed
npm ERR! There is likely additional logging output above.
npm ERR! System Darwin 13.3.0
npm ERR! command "node" "/usr/local/bin/npm" "run" "preprocess-test"
npm ERR! cwd /Users/MoOx/Sync/Development/forks/components-flex-embed
npm ERR! node -v v0.10.29
npm ERR! npm -v 1.4.16
npm ERR! code ELIFECYCLE
npm ERR! 
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/MoOx/Sync/Development/forks/components-flex-embed/npm-debug.log
npm ERR! not ok code 0

npm ERR! suitcss-components-flex-embed@1.7.1 build-test: `npm run setup && npm run preprocess-test`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the suitcss-components-flex-embed@1.7.1 build-test script.
npm ERR! This is most likely a problem with the suitcss-components-flex-embed package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     npm run setup && npm run preprocess-test
npm ERR! You can get their info via:
npm ERR!     npm owner ls suitcss-components-flex-embed
npm ERR! There is likely additional logging output above.
npm ERR! System Darwin 13.3.0
npm ERR! command "node" "/usr/local/bin/npm" "run" "build-test"
npm ERR! cwd /Users/MoOx/Sync/Development/forks/components-flex-embed
npm ERR! node -v v0.10.29
npm ERR! npm -v 1.4.16
npm ERR! code ELIFECYCLE
npm ERR! 
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/MoOx/Sync/Development/forks/components-flex-embed/npm-debug.log
npm ERR! not ok code 0
```