Add new test type wdspec to manifest generator

This patch adds a new test type, wdspec, short for WebDriver
specification tests to the manifest generator.

It also makes a number of improvements to tools/scripts/manifest.py.
fixup! Add new test type wdspec to manifest generator
fixup! Add new test type wdspec to manifest generator
fixup! Add new test type wdspec to manifest generator
fixup! Add new test type wdspec to manifest generator
fixup! Add new test type wdspec to manifest generator
fixup! Add new test type wdspec to manifest generator
Merge pull request #1094 from andreastt/ato/manifest_webdriver

Add new test type wdspec to manifest generator