Assert context uniqueness against merged config

Prior to this commit, the uniqueness check for @ContextConfiguration
attributes within a @ContextHierarchy was performed at a single test
class level instead of against the merged configuration for all test
class levels in the test class hierarchy.

This commit addresses this issue by moving the uniqueness check
algorithm from resolveContextHierarchyAttributes() to
buildContextHierarchyMap() within ContextLoaderUtils.

Issue: SPR-10997