debian/tests: skip flakey TestNumpyFunc3.test_interp tests

TestNumpyFunc3.test_interp is flakey at levels 6,7,8 especially with
complex numbers (_7c etc) and depending on BLAS implementation.

i386 is flakey also in the real variant, so skip all test_interp_6,
_7, _8.

See https://github.com/serge-sans-paille/pythran/issues/2000