This reverts commit 62765608.
as this is breaking on boost < 1.73

ref. https://github.com/boostorg/test/commit/235acea2fa4b1eb7580acbfcce94d696c86bb207#diff-9131a06d2a1a6d0c877c9b7909acbbde

Add entity test to check writeCompletionList + license modification.

Testing and checking output of writeGraph.

    g_pool and g_factory global variables have been replaced by static methods
    getInstance in each class.
    getInstance returns a pointer to the unique instance of the class and
    creates it if needed.

Corrected a segfault problem in the test pool. The problem at the global level remains. At least, the test passes now.