tests: iptables-test: Fix for 'make distcheck'

tests: iptables-test: Fix for 'make distcheck'

This was a tricky one: Since called from VPATH topdir, extensions/ do
not contain test files at all. The script consequently passed since 0
tests failed (of 0 in total).

Fix this by introducing TESTS_PATH which is extensions/ below the directory
of the running iptables-test.py. Keep EXTENSIONS_PATH as-is: The built
extensions are indeed there and XTABLES_LIBDIR must point to them.

Signed-off-by: Phil Sutter <phil@nwl.cc>