use bbcov to find reduced test set for the same code coverage

_From [zhao...@google.com](https://code.google.com/u/106321947286816917100/) on April 23, 2013 11:12:20_

bbcov is used for code coverage, we can extend its feature to support finding reduced test set with the same code coverage.

The basic idea is to run all the possible tests with bbcov and produce all the log files. Then on bbcov2lcov processing log files, we report which log files add new basic block and which do not. Thus all the log files (and its corresponding runs) that adding new basic blocks are the reduced set.

To be note:
1. It is not a minimal set, but only a reduced set.
2. The reduced set only has the same basic block coverage.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1148_