Format test_definition.xsd by `xmllint --format`

Reference: http://xmlsoft.org/xmllint.html

This just formats the test_definition.xsd indent with 2 spaces,
and removes empty line.
No code logic change.
Merge pull request #49 from zqzhang/zqzhang/xsd-format

Format test_definition.xsd by `xmllint --format`