Move io.bitsquare.{=>app.}BitsquareEnvironmentTests
Polish and test BitsquareEnvironment

 - Introduce a test-time dependency on spring-test module for access to
   MockPropertySource and friends.

 - Add BitsquareEnvironmentTests and test that property source precedence
   works as expected, i.e. that properties supplied on the command line
   have highest precedence, overriding those picked up via environment
   variables, system properties, the bitsquare.properties file or any of
   the other available property sources.