ACCUMULO-2470 Unit tests for server/base module

This commit adds unit tests to code under the server/base module. It also includes changes
to o.a.a.server.problems.ProblemReport to enable comprehensive testing.
Merge branch '1.6.0-SNAPSHOT'

Conflicts:
	server/base/src/main/java/org/apache/accumulo/server/problems/ProblemReport.java