Add checks for READMEs for each virtual test suite.

By convention, each virtual test suite should have a README.txt documenting
what directory it mirrors and what command line args are needed, so that
you can tell what an "empty" directory under LayoutTests/ is for.

This patch modifies lint-test-expectations to check that those READMEs
exist, and will help avoid the possibility that we'll delete whole directories
thinking they are unneeded and introduce weird errors when TestExpectations
still refer to nonexistent directories or files.

R=ojan@chromium.org
BUG=369385

Review URL: https://codereview.chromium.org/267803007

git-svn-id: svn://svn.chromium.org/blink/trunk@175748 bbb929c8-8fbe-4397-9dbb-9b2b20218538