Make application context for test framework more consistent with runtime

Augment the Spring application context for the GeoServer test framework to add in bean refs and properties derived from the environment and mock ServletContext.  This will allow various builtIn bean references that are available at runtime to be available during testing (e.g. `"servletContext"`).  Code was grabbed [the spring source](pulled for https://github.com/spring-projects/spring-framework/blob/3.1.x/org.springframework.web/src/main/java/org/springframework/web/context/support/AbstractRefreshableWebApplicationContext.java#L153)...