Enable sharding of individual testsuites in tools/test.py

Enable sharding of individual testsuites in tools/test.py

This patch enables two new flags for the tools/test.py script;
--shard-count - giving the ability to split the tests to be run
into shard-count chunks.
--shard-run - giving the ability to specify which of the shards to actually run.

Example
  tools/test.py -j15 --shard-count=2 --shard-run=1 mozilla
would split the mozilla tests into two chunks and run the tests in the first chunk

Running:
  tools/test.py -j15 --shard-count=2 --shard-run=1 mozilla
  tools/test.py -j15 --shard-count=2 --shard-run=2 mozilla
is equivalent (in terms of test coverage) of just running:
  tools/test.py -j15 mozilla

In addition, tests are now sorted before they are returned from the
test specific ListTests methods (sputnik and mozilla tests where
already sorted before they where returned).

This change is needed to split a single test suite over two slaves on
the waterfall.

Review URL: http://codereview.chromium.org/6127003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6248 ce2b1a6d-e550-0410-aec6-3dcde31c8c00