Improve and redesign test programs + unify their output (ticket #25)
- Move graph related utilities form test_tools.h to graph_test.h.
- Move the contents of graph_utils_test.h to graph_utils_test.cc.
- Rename map_test.h -> graph_maps_test.h.
- Rename digraph_test.h -> graph_test.h.
- Many improvements in the following files:
* digraph_test.cc
* graph_test.cc
* graph_test.h
* graph_maps_test.h
* graph_utils_test.cc
* bfs_test.cc
* dfs_test.cc
* counter_test.cc
- Test programs print messages only if it really seems necessary.
- Remove \file commands form .cc test files.

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