valgrind: disable avx_estimate_insn.vgtest

This test fails consistently on the new valkyrie autobuilder cluster

The estimate instructions (rcpss, rcpps, rsqrtps, rsqrtss) are, as the
name suggests, not expected to give a fully accurate result. They may
produce slighly different results on different CPU families because
their results are not defined by the IEEE standard.

(From OE-Core rev: 9269edb63d3c68de62326f5960ffff0c1068e709)

Signed-off-by: Steve Sakoman <steve@sakoman.com>