tests: simplify Makefile.am now that libdbus is always dynamically linked

tests: simplify Makefile.am now that libdbus is always dynamically linked

testutils_shared_if_possible_cppflags is now just a copy of AM_CPPFLAGS,
which is the default and does not need to be given explicitly, so
those lines can be removed.

Similarly, testutils_shared_if_possible_libs is just the
libdbus-testutils.la convenience library, so expand it and
remove the unnecessary variable.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=83115
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>