Speeding up Xapian's testsuite

1. Remove backendmanager_local, make the tests run on databases with remote + local shards.

2. Implement timer to track slow testcases.

3. Add support for generated databases in backendmanager_remoteprog, backendmanager_remotetcp and backendmanager_honey.

4. Try and Fix bugs generated by the wider test coverage.

5. Seperate testcases by respective backends and run them parallely, and output the results in TAP format.