[tests/tools] Don't install libdisable-npots.so

[tests/tools] Don't install libdisable-npots.so

The libdisable-npots library is just used as a helper as part of
make test so it should not be installed.

If noinst_* is used then automake will generate a static library but
this won't work with LD_PRELOAD so we then need an extra custom rule
to link that into a shared library. The custom rule uses the $(LINK)
Makefile var which gets put in the Makefile because of the static
library. We pass libtool a stub -rpath option which causes it to
generate a shared library.