This commit updates the precedence of properties configured using
@SpringBootTest to align with @TestPropertySource. Properties configured
using properties on @SpringBootTest are now added to the same property
source as those configured using properties on @TestPropertySource so
the precedence described in the javadoc of @TestPropertySource now
applies in full. Additionally, if both @TestPropertySource properties
and @SpringBootTest properties configure the same property, the value
from @TestPropertySource will win.

Closes gh-4828