Add TestingEnvironmentConfigurator (#364).

To configure the testing environment, implement
"TestingEnvironmentConfigurator" class and put
the implementation class name in
"META-INF/services/com.asakusafw.testdriver.core.TestingEnvironmentConfigurator".

Clients can suppress this feature with setting
"-Dasakusa.testdriver.configurator=false".
Merge remote-tracking branch 'private/issue/364' into 0.6-develop