Explode wtf/tests

Rather than putting all the unit tests in one folder, the current style is to
match Chromium and put the unit tests next to the code that they test. As a
side benefit, I won't typo text/test or mistake the testing StringImpl.cpp for
the normal StringImpl.cpp when usting Ctrl-P in Sublime anymore.

R=jamesr

Review URL: https://codereview.chromium.org/25414004

git-svn-id: svn://svn.chromium.org/blink/trunk@158668 bbb929c8-8fbe-4397-9dbb-9b2b20218538