samples: tfm: Disable initial attestation test

Disables the initial attestation suite, which we are unable to run
at present due to a license issue with the underlying QCBOR library
which this service depends on in TF-M.

This commit can be reverted once an acceptable solution has been found
for the QCBOR license in TF-M.

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>