We use some internal Mockito classes and some  breaking API changes
have been made to them in Mockito 2. This commit introduces a utility
class, SpringBootMockUtil, to shield our code from these differences.
Mockito 1 is called directly and Mockito 2 is called via reflection.

To allow these changes to be tested, FilteredClassPathRunner has been
enhanced to also support overriding a dependency on the class path.
As a result it has been renamed to ModifiedClassPathRunner. The new
ClassPathOverrides annotation can be used to provide the Maven
coordinates of one or more dependencies that should be resolved and
added to the class path. Such additions are added to the start of
the class path so that they override any existing dependency that
contains the same classes.

Closes gh-6520