Previously, SpringBootContextLoader configured the environment with the
test property sources using an ApplicationContextInitializer. This was
because TestPropertySourceUtils did not provide a method to directly
configure the environment using properties files, it had to be done
via an application context. An unwanted side-effect of this was that
the test property sources were not being configured before
ConfigFileApplicationListener examined the environment to determine the
name and locations of the files that it should be loading.

This commit takes advantage of a new method that was added to
TestPropertySourceUtils which allows properties files to be added
directly to the environment without using an application context. This
means that the use of the ApplicationContextInitializer can be removed
and the test property sources can be applied to the environment before
the application context is created.

Closes gh-5728