Warn if no default test is defined in benchmark_config

If there is no test named "default" this prints a warning. `pstef` reported this on freenode, because he/she was having problems with `toolset/run-tests.py --test hhvm`. Turned out that HHVM only defines a test raw, so you had to say  `toolset/run-tests.py --test hhvm-raw`. It seems to me that there should always be a default you can run by referring to the framework by name. 

List of current offenders: 
```
WARNING:root:Framework lwan does not define a default test in benchmark_config
WARNING:root:Framework ULib does not define a default test in benchmark_config
WARNING:root:Framework servicestack does not define a default test in benchmark_config
WARNING:root:Framework redstone does not define a default test in benchmark_config
WARNING:root:Framework start does not define a default test in benchmark_config
WARNING:root:Framework stream does not define a default test in benchmark_config
WARNING:root:Framework play2 does not define a default test in benchmark_config
WARNING:root:Framework wildfly-ee7 does not define a default test in benchmark_config
WARNING:root:Framework silica does not define a default test in benchmark_config
WARNING:root:Framework yaf does not define a default test in benchmark_config
WARNING:root:Framework pyramid does not define a default test in benchmark_config
WARNING:root:Framework grape does not define a default test in benchmark_config
WARNING:root:Framework padrino does not define a default test in benchmark_config
WARNING:root:Framework rack does not define a default test in benchmark_config
WARNING:root:Framework rails does not define a default test in benchmark_config
WARNING:root:Framework rails-stripped does not define a default test in benchmark_config
WARNING:root:Framework sinatra does not define a default test in benchmark_config
WARNING:root:Framework play2 does not define a default test in benchmark_config
```