Define a base TestException class

Define a base TestException class

A new base class DPL::Test::TestException is defined as base for
 * DPL::Test::TestFailed (prev. DPL::Test::TestRunner::TestFailed),
 * DPL::Test::TestIgnored (prev. DPL::Test::TestRunner::Ignored).

All three classes were moved into separate files.

Common base class simplifies handling of exceptions that can be
thrown during tests.

Change-Id: I1fadb09b7781bf22a0090043a46ca48c55c9962b