[misc] Update WebAudio test purpose to reflect XWALK-1993

- Reword the README.md
- Remove trailing space from pack.sh
[misc] Format testcase.xsl and testresult.xsl

- Apply xmllint --format
- Replace tab with 2 spaces for indent
[misc] Beautify the WebAudio test case

- Indent index.html by 2 spaces
- Refine the test purpose, step, and expected result
- Move samples to res/ as that in behavior tests
Merge pull request #537 from zqzhang/zqzhang/xwalk-1993

[misc] Beautify the files in xwalk-system-tests