Fix Java-specific Doxygen unit tests.

Put these tests in DOXYGEN_TEST_CASES and not just CPP_TEST_CASES as the
"-doxygen" switch is only used for those in the former and not the latter.
Fix running Doxygen Java unit tests from separate build directory.

Use full path for CommentsParser.java instead of relying on it being found by
"find . -name *.java".

Also use tools.jar from the JDL for the Doxygen-specific tests only, the other
ones don't need it.
Work around apparent clang bug.

Clang seems to be confused by "tm" being the name both of a local variable and
a struct tm from the standard <time.h> header, included indirectly in the
doxygen branch from java.cxx via "swig.h" and <stdlin.h>.

Just removing the unnecessary cast helps to clear the confusion.
Make Doxygen unit test supporting module Python3-friendly.

Add parentheses around print argument to make the same code work in both
Python2 and Python3.